Most of what I have in this guide is still accurate. Follow Through was pretty much the only major change (other than a meta shift and then another meta shift a few days later...) in terms of actual talent changes, and since all my advice is situational, there's really not that much I would have to say.

tl;dr of Kael patch: If there's a Kael'thas, take Sixth Sense at 13, no question at all. If there isn't, but there's a couple tanks (yay, two and three tank meta...), consider Giant Killer.

That said, I've recently been spending more time attempting to go competitive as the main support than as assassin, so my time to mess around with builds on Illidan is very limited at best. This guide will likely stay out of date, and as such I've updated the title to reflect that. If this ever changes, this guide will be the first thing I update, but that may be a significant time from now.

This guide is not for the new player just learning Illidan (unless you just really like theorycrafting). In writing this, I assume you have enough Illidan games under your belt to feel mostly comfortable with him, and simply want opinions on talents you don't normally take. If you're new to Illidan, I strongly recommend Severage's Illidan guide. That's what I used when starting him, and it has pretty much all the information a beginner could want.

This guide is for people who have played around with Illidan and are open-minded to potentially non-meta talents. I feel Illidan has a lot more viable talents than most people give him credit for, and in writing this, I hope that people may learn to be more open-minded when someone takes a talent they may not see as often.

Also worth noting is that my experience with Illidan comes from mid to high level play. Some of what I say may or may not be relevant the lower you go in MMR (particularly if it's decision-making rather than mechanics that is the problem, as I used to struggle with), but on the flip side, it can't hurt to at least read the theory. This guide is also largely irrelevant if you're at the pro level, but let's be real, you're not going to be reading it if you're there anyway.

So why you?

Because I'm the world's best and most humble Skillidan, clearly.

No, but seriously, I've started varying my build greatly based on what match-up I'm in, and I've spent numerous games refining and testing what works and what doesn't. I've reached Rank 1 playing mostly Illidan, and on my accounts combined, I have 350+ Illidan games. I've been told by many, many people (some friends, some random) that I'm one of the best Illidans they've seen, and I've had quite a few people politely approach me after a game and ask me why I took a talent that I did. This is my attempt to have somewhere solid to point them to instead of saying the same things to every person with less detail.

tl;dr
Shadow Shield is the standard. Some games, particularly on Garden of Terror, Seasoned Marksman is a good alternative if you feel you'll be able to survive early game.


Offensive Options

Battered Assault is a hard talent to justify. If you're really after a damage increase, Seasoned Marksman ramps up quickly enough to render Battered Assault irrelevant in all but the shortest of games.

Seasoned Marksman can be good for games you expect to go long, as long as you know you can survive early without too significant losses. If you're on a snowbally map, the enemy has a bunch of lane bullies, or you just generally feel you'll be falling too far behind in EXP, please don't take this talent. That said, I absolutely love this talent specifically on Tomb of the Spider Queen and Garden of Terror, provided the enemy doesn't have absurdly high early burst.

GoT Specific SM:
If you pick up the plant, every time you drop a bulb on a minion wave, those will count for SM stacks. I'm sure you can see the antics in how quickly this can allow you to ramp up, particularly if the enemy team is attentive and you end up running from lane to lane dropping potted plants. Taking the terror on Illidan can also be great in a teamfight situation, since normally the enemy team would want to burst you down, but in this regard, they've got to get the terror down first before a full HP Illidan pops out and (probably) makes their day miserable.

That said, please do not use the terror irresponsibly. The plant does not exist purely as a stack generating machine, and you should use it to the team's benefit before your own. If doing so aligns with getting stacks, all the better (and it often does), but don't just ignore fights and structures to run around stacking.

TSQ Specific SM:
You're going to be too squishy to do real ganking, so you're mostly going to be roaming between lanes farming stacks.

INFORM YOUR TEAM THAT YOU PLAN TO ROAM FOR SM STACKS. Seriously. Communication is the absolute key here. If your ally sees you running into lane and doesn't realize you're mostly after stacks, this can cause some really awkward situations and heavy losses. That said, I've had games where I'm at easily above 30 stacks around the 10 minute mark just roaming between all three lanes with good clearers. It's gimicky, but if you can pull it off, it pays off heavily.

Defensive Options

Shadow Shield is good every game. Seriously, when in doubt, take Shadow Shield. SS helps you with mercs, helps you with hero killing, helps you with looking good in front of your friends, and even helps restore hair on balding old men. It's an all around great talent.

Don't take Regeneration Master. Just don't. The reason Seasoned Marksman is viable as a stacking talent is because you're not taking it for your survivability, and it makes you a late game monster. Shadow Shield adds up to roughly the same amount of protection in a teamfight as Regen Master, but is significantly stronger in the early game where you need the most help anyway. Where Regen Master shines is after a teamfight or skirmish is over and you're limping your way back to lane/another objective -- except that once you get there, your passive's lifegain will do the job well enough as-is (which is assuming you don't have Fountain up to drink from anyway).

tl;dr
Immolation is the standard. Many games, particularly when you're fighting strong AoE healers, Marked for Death is a good option. Alternatively, Thirsting Blade synergizes nicely with Seasoned Marksman, if you took that. Don't take Fel Reach. Ever.

Offensive Options

This is where it starts to get interesting. Immolation is a great all-around talent, and most people will take this every game. There's not really a downfall to that, as it's particularly useful when trying to take down Bruiser camps on any map, skull camps in the Mines, or seed camps on Garden of Terror.

Another fairly meta talent is Thirsting Blade. Most people will only take this in combination with Seasoned Marksman, but there are some fringe cases where I'd honestly recommend it over Immolation. If their team has high sustained damage (think Tychus with Drill, Valla with auto attack build, Sylvanas with Follow Through, or Nazeebo with just about anything) that you know you're going to have trouble surviving, I'd consider taking Thirsting Blade. I've found that having just that liiittle bit of extra survivability often makes the difference in fights against those heroes.

And the final viable option, and probably the most frequently questioned talent that I take -- Marked for Death. Originally, I scoffed at people who took this talent, but recently, I've found myself taking it more and more. Immolation adds up faster in terms of sheer numbers on the stats board, but Marked for Death can allow you to burst a single priority target more quickly. Mostly I take this against enemy teams with strong AoE healing that may be lacking the initial burst to keep someone up. Good examples of these teams are anything that's relying on Malfurion without a back-up healer, or any double support comp that doesn't have Rehgar (or specifically Tassadar+Tyrande combo, where I'd take Immolation over MFD just because they're both off-healers with no AoE to speak of at all).

Note: Marked for Death can be especially attractive when fighting Jaina. One of the reasons I attribute my high rate of success vs. her compared to the average win rate where she's considered an Illidan counter is that I stay very, very active, and most Jaina players can't reliably land a combo against an Illidan that's jumping around constantly. Not only does MFD work well with a very active playstyle, but it helps burst her even faster since she's already quite fragile.

Don't take Fel Reach. Just don't. A kitten is brutally murdered every time someone takes this talent. Fel Reach is one of the only talents in the game that legitimately makes a hero worse. Illidan doesn't have trouble staying on targets. Everything in his kit is designed for that. Fel Reach would be viable if increased the range on Sweeping Strike and let you CHOOSE the distance you traveled, but as it is, it only extends it by a set length, so more often than not, you're going to overshoot your target and just look silly. If you're talent gated, take Marked for Death over this one. Please.

tl;dr
First Aid in 99.999% of games.

Offensive Options

One important thing before I even get partially into offensive options -- if you're having trouble surviving, take a defensive option. A dead Illidan does no damage regardless of how perfectly mathcrafted and synergistic his total offensive build talents are. Most offensive options should only even be considered when you have double healer, and the enemy doesn't have a high-damage team (no: Jaina, Sylvanas, Tychus, Burst-atul, etc.)

Follow Through sounds alright on paper, but you're not always going to be able to weave your CDs. That said, if they have a SUPER immobile team (people like Gazlowe, Raynor, Malfurion), you can potentially get some mileage out of this. First Aid is usually still better unless you're overflowing with heals anyway, but this is about as close as you'll get to ever picking FT on anything but Sylvanas.

Defensive Options

First Aid is love, First Aid is life. Worship First Aid, and it shall forever serve you well. No, but seriously, First Aid is never a bad talent. If you go Metamorph, you can bait a surprising amount with FA, though you probably know that by the time you've read this guide. For that reason, I'm not going to go too in-depth, as this should be pretty much a self-explanatory talent.

Reflexive Block is.. Questionable. If you're going 1v1 Huntidan, this can be almost viable when wanting to take down auto attacking heroes, though First Aid is generally better as an all-around viable talent. If Block negated 100% of the damage, or you got a few more charges, this might be a more viable talent, but as it is, it's hardly worthwhile.

Utility Options

Thrill of Battle is probably the most viable of the non-First Aid options, as it functions as both offense and defense. If you take this talent, pop it early and often because of its low CD, even if you're just farming a wave/hitting an uncontested structure. I admittedly don't take this as often as I perhaps should, since I rarely deviate from FA, but this is probably the most viable of the lot after it.

Rapid Chase is mostly pointless. If you've just Q'd someone, you're probably not going to need help catching up to them unless they've used Sprint. If they've Sprinted, you're not going to catch them with this unless you would have to begin with. If you've Q'd someone to get out, the 20% speed isn't going to do that much more than just running/ulting out/W'ing out. This talent is just too low impact compared to oh, every single other option on this tier.

tl;dr Metamorphosis for most games. The Hunt is sometimes viable as an off-option some games, but there's a lot to keep in mind with it.

Metamorphosis
Metamorphosis is a great teamfighting ability, and since nearly everyone uses it, I don't think I need to go into too much depth. The ability to gain invulnerability frames while taking focus and come back out with bonus stats is ridiculously valuable, and if you feel your team is going to be teamfighting frequently (so basically, most games), this is probably the heroic you want to take.

Hunt
The Hunt is generally considered a less viable talent for many reasons. Metamorph gives you significantly less teamfight presence, Hunt has a fair few bugs that can result in your death, Hunt takes strong map awareness (is that person REALLY alone, or is their team close enough to help?), etc. That said, it's not an unpickable talent.

If your team is behind on a large map, and the enemy isn't completely deathballing, Hunt can be viable for getting back into the game (in unorganized play). I can't tell you the number of times I've helped my team claw back into the game by slowly breaking down the exp lead with picks on maps like Cursed Hollow. That said, The Hunt tends to fall off significantly the farther into the game you go, as teams tend to split less, and you thus have fewer opportunities for picks. Split pushing and hoping to draw someone off is all well and good, but if your team can get wiped (or even just 1 key player) in the time it takes you to fly across the map, you're in deep trouble.

If there's an obnoxious split-pushing hero ( Gazlowe, backdoor Abathur, Sylvanas), Hunt can be viable for shutting them down. Keep in mind I very specifically did NOT list Azmodan there -- a half-skilled Azmodan can easily solo you and then keep pushing as he laughs his way to your mercenary camps/tower/fort/keep/core/whatever. Hint: If there's a backdoor Abathur, you can hit CTRL + the F5-F8 to save camera positions. Save one for each usual teleport spot and keep an eye on your base, and you'll be able to quickly ruin his day the moment he shows up in your base.

If the enemy team has a channeled ability and your team has 0 CC, it can be viable to take Hunt to shut them down. It's still questionable, and you have to be careful of your positioning so your interrupt doesn't get interrupted, but depending on the game, that may be more important than the raw stats that Metamorphosis provides. Abilities this applies to are ones like
Li Li's Jug of 1,000 Cups
Nazeebo's Ravenous Spirit
Tyrael's Sanctification (if he ever takes it..)
Valla's Strafe
Pretty much any other ability can be stopped or blocked in some other way, and probably isn't worth picking up Hunt for.

tl;dr
Sixth Sense vs. high ability dmg heroes.
Giant Killer vs. teams you're not getting instagibbed in.

Offensive Options

Giant Killer is generally considered the meta talent on this tier. It's great for whittling down tankier heroes (and pretty much NECESSARY for some of the more obnoxious ones like Diablo and Chen), while still giving you a small damage boost against even paper-thin heroes. If you think you can survive the enemy team (or if they have AA-based burst), this is generally a good option.

Lunge is pretty much irrelevant. 30% isn't a big enough boost to do much other than leap in too early and die. You shouldn't really be having sticking problems against most foes as Illidan, and this talent isn't going to do enough to fix any that you're having.

Defensive Options

Sixth Sense is a great talent against heroes that really want to mess up your day with abilites. Pop this before a Kerrigan jumps your face and you've renedered her mostly useless (though a well-timed Q or pre-emptive W does the same). This talent also makes it much easier to survive burst from Jaina, Magestad, Azmodan, etc. This talent is borderline required when fighting a Drill Tychus, and generally recommended vs him no matter what he picks.

Friend or Foe falls somewhere between 'of questionable use' and 'why does this even exist.' One of the things you may noticed after playing Illidan a few times is that if you Dive a unit as it dies, there's a fraction of a second where you're forced to sit there and wait until you can move again since you can't actually flip. This talent has the same problem. If you Dive to an ally, you get stopped momentarily since you can't flip. It takes a lot of the fluidity out of Illidan, and you'd arguably be in a much better position if you had just W'd from whatever threat you were concerned about, or Q'd to a different enemy.

Utility Options

Unbound is an interesting talent in a lot of ways, so it's a shame it's on this tier instead of a different one. I sometimes pick up Unbound on the twistier maps, as there's so many good nooks and crannies you can utilize on maps like Cursed Hollow, or to a lesser extent Garden of Terror/Dragon Shire. This talent can also be used as a way of making Q'ing past a game safer, but by the time you get this, it tends to be getting less and less relevant in that regard. If this talent restored Unstoppable frames, it would absolutely be worth considering over the other options, but for now, it's relegated to 'niche pick depending on map and only if they don't have some of the popular meta picks right now.'

tl;dr

Blood for Blood if you're feeling safe and/or they're extremely high in HP caps. Stoneskin if you're getting wrecked.

Offensive Options

you thought blood for blood was going to be here but it's in utility because it's even better than that.

Defensive Options

Stoneskin can be a huge life-saver when paired with Metamorphosis. If you're taking Hunt, it's probably not worth taking, but if you land a 3-5 man ult and pop Stoneskin, it's going to be much harder to take you down than people normally expect it to be. This is particularly useful when fighting high burst and/or wombo comps that run out of options if they haven't killed you in a combo or two.

Utility Options

Blood for Blood is an amazing talent. Although initially it looks like an offensive option, there are so many uses for it that I have a hard time not quantifying it as Utility. Running away and getting chased? BFB to slow them down and gtfo. Chasing someone with Sprint? Pop BFB and enjoy. Enemy team has a Rehgar? Hold BFB (depending on how many BFBs your team has) until the enemy is at 15% to bait the Rehgar into holding Ancestral too long (seriously, the number of fights we've won off Ancestral going on a 9s cd due to BFB execute is ridiculous).

Hunter's Onslaught is pretty much not worth picking. Your abilities aren't the bulk of your damage, and 15% is just too little to add up anywhere near quickly enough to justify this as a talent. BFB is going to return more health when you need it most of the time, and Stoneskin will just flat-out prevent the damage. The only time I could even THINK about this as a talent is if they have some weird quickmatch team like BW + Li Li + Illidan + Muradin where you're just going a pure ability build because you've been entirely screwed over by hero picks. Even then, though, it's arguable whether that's truly better than just missing/getting evaded constantly.

Second Sweep is a talent I'd love to say is worth it when combined with Unbound, but it's really just not. In most situations, you're already probably going to be getting Sweeping Strike up often enough to use it as much as you need in teamfights, and the number of times you escape because you've W'd twice compared to once vs. W'ing once and then BFB/Stoneskin is.. well, it's just not going to happen.

tl;dr

there is no tl;dr because everything on this tier is viable.

Nowhere to Hide
Nowhere to Hide is obviously only available when you go Hunt. If you've already thrown yourself into this role, and you don't think you're going to have a hard time killing whatever you're Hunting, you might as well go for this. That said, keep in mind what you've been hunting primarily, because if they have access to Bolt + escape abilities, taking this over Bolt may cost you.

Demonic Form
Demonic Form is sometimes overrated. Don't get me wrong; it's a fantastic ability in most situations, but it's not quite as much of a catch-all talent many people think it is. Against teams where you're getting stuck in CCs, it can absolutely be the right choice, but I've seen many games where people pick it up because they're getting burst despite not getting CC'd at all. That bonus health is not permanent, and the 20% extra attack speed doesn't boost your survivability as much as you might think.

Nexus Blades
Nexus Blades is a talent I used to undervalue significantly, admittedly. There's pretty much nothing that can run from an Illidan running Nexus Blades (though Bolt + Sprint/escape ability of choice can be another matter), and that includes slippery heroes that otherwise could like Rehgar or Li Li. Generally, this talent is a luxury pick that you can take if you're super far ahead, or don't feel like Bolt is necessary/the enemy doesn't have many bolt heroes you're going to want to be chasing (see list below). If you're getting CCed or bursted down, do not pick this talent.

Bolt of the Storm
Bolt of the Storm is never an objectively bad pick. The uses for it are endless, whether that be repositioning in a fight, chasing, running, dodging a ground-targeted/skillshot ability, or any of a number of other uses. There's very little that's unique or requires in-depth knowledge of with this talent, since you've probably used it on other heroes as well.

Heroes that can get Bolt of the Storm for your reference if your team levels to 20 first: Azmodan, Chen, Diablo, E.T.C., Illidan, Jaina, Kerrigan, Malfurion, Murky, Nazeebo, Nova, Raynor, Stitches, Sylvanas, Thrall, Tychus, Valla, Zagara
Not all of those take it frequently, but it's good to keep in mind who can and can't take it.

coming eventually when I figure out how I want to format this section...