Falstad is my favorite hero in HOTS, and I have played him to level 12. I have a 50% win rate but a 75% winrate in my last 100 games with him. This reflects the fact that Falstad is a high skill cap hero and also how I have improved my builds. I am now sharing what I've learned with you. I believe that Falstad should always be built to emphasis his autos, not his mage abilities. He has strong autos and mana issues.

Falstad is a master of strategy. Use Flight to attack weakness and Mighty Gust to retreat from strength.

Attack weakness, retreat from strength. Attack weakness, retreat from strength. Observe these two and become a winner.

Use Hammerang for waveclear and Flight to set up sneaky ganks.

To maximize waveclear, shoot the rang as the minions walk up single-file. You can hit all 7 minions with both halves of the rang if you do this. After they spread in the normal H pattern you can still hit 6 of 7 with careful rang placement. Use rang at most once per wave, then stay behind your minions and autoattack. Whenever a hero comes in range make sure to get in your poke, but there's no reason to overextended to increase poke.

Pay attention to other lanes. Falstad's Flight ability gives him massive gank potential and you don't ever have to miss a wave of exp to do it. As you waveclear with Q, look for lanes with enemies that have been damaged. When you find a likely target, ping the map so your lane ally knows you're coming, and then use Flight to fly into the bushes that flank them. The essence of this gank strategy is surprise. Don't start the flight when the enemy has vision on you - clear your wave, back off a few steps, then fly. Plan your flight so that you do not travel through or arrive in enemy vision. The first time your target should know you're stalking them is when the rang flies out of the bush and slows them.

There are two other ways to use Flight during lanes. One is to change the odds of an engage that one of the other lanes is involved in. Whenever an allied lane engages in an even fight, try to fly there. You can get there before enemy reinforcements and end the fight before anyone else shows up. During engages you pick your flight destination based on what hurts the enemy most. Often times this is flying directly between them and their tower. They will see you coming because your flight paints a white circle on your destination, but if you plan this correctly they won't be able to avoid you, or will be forced off of their retreat path.

Finally if there is no good action for early ganks, you can use Flight instead of drinking the well. Hearth back and fly back into the action, keeping your well cooldown for more urgent moments.

Note that if you're not planning to use Flight to pressure the enemy lanes with ganks, then you shouldn't play Falstad, because without Flight he's just an inferior Valla. Falstad is all about being in the right place at the right time.

Falstad always duels the same way.

Lead with Hammerang, drop Lightning Rod, and stutter-step autoattack. Stutter towards them as they try to flee, or away from them as they try to close with you. You should stutter step in every fight, even if it's not necessary. This will get you in the habit and improve your skill until it's second nature to you.

Save your Barrell Roll as an escape. A rule of thumb is that 90% of your Barrel Rolls should be escapes and 10% should be to clinch a kill. Don't forget that Barrel Roll goes over impassable terrain, and this makes it one of the best escapes in the game, up there with Banshees and Dwarf Toss.

If you are going to lose the fight and you notice soon enough, use Flight to get out of there. It takes 1 second to begin flight and you can still be attacked in the air, but many escapes are possible this way.

Hang back and poke with Hammerang and stutterstep aa's until a hard engage. Use Lighting Rod when you will get multiple ticks out of it. It's ok to use it on a tank when no other targets are available. This is especially true when you are playing with Charged Up and Static Shield. In that case you want all six ticks to get the shields, so tanks are often the best target. Barrel Roll is for escapes. Flight can be used to clean up a runner, never Barrel Roll! (Once you are pro with Falstad the Barrel Roll rule no longer applies).

On many maps you can use Flight to clean up a runner that's hearthing between a fort and a keep, then Barrel Roll over a wall to safety. This relies on superb awareness of the enemy team.

Finally, use Mighty Gust liberally to disengage, split the enemy team, or in some cases, to push some squishies into a death ball. It has a low cooldown. If you can use it to save 1 teammate it is worth doing. It is the most decisive disengage in the game. Retreat from losing battles!

T1.
Seasoned Marksman

Figure that you get 4-10 AA's and 1-2 rang hits in any hero-on-hero engage. This means you need to stack Gathering Storm about 3-4 times higher for it to be on equal footing with Seasoned Marksman (and SM will always be better at killing boss minions). I rarely get 3x as high even if I take Flow Rider, which I never do any more because Falstad has mana issues and I much prefer to use my Flight for ganks than for mana. In my games, SM just does more damage than GS, even in teamfight-heavy, low farm map such as Battlefields and Cursed Hollow.

I have only seen Updraft used once, by k1pro in C9's semifinals against DK. I don't think the 30% extra roll often makes a difference, although of course sometimes it will. Finally, I never use Bribe, but it might actually be a good talent in some maps, e.g., Blackheart's Bay on Siege Ogres, Cursed Hollow on Siege Ogres right as a curse pops, and Battlefields on the shaman camp.

T2.

Charged Up is a skill I take against sustain comps and bruiser comps with the intention of taking Static Shield at 13. I learned this trick by watching k1pro. With Charged Up and Static Shield you can win a heads-up duel with any assassin in the game. It will usually be my pick against Tychus, Kerrigan, Sonya, Thrall, The Butcher, Tyrael, Valla, Falstad, Raynor, Kael'thas, Jaina, Sgt. Hammer, Nazeebo. I also pick this when divers and bruisers are on my team, so that I can follow them into the fray. 35% life as shields is a huge amount on a 15 second cooldown. (Compare to First Aid).

Flow Rider is not viable due to Falstad's mana issues. You just don't have the mana to cast more abilities. If Falstad had double the mana, then Flow Rider would be reasonable, but it is not right now.

Vampiric Assault is awesome for keeping you topped up. I take it when I don't have a healer or when I am facing high burst, e.g. from enemy Nova. Yes it doesn't save you in the moment, but you can enter fights at 100% and getting poked doesn't send you back. I pick this against Nova, Lunara, Zeratul, Cho'Gall, and other pokey characters, or when we don't have a lot of healing. If you team is dive heavy and they have a pokey comp, take Charged Up now and First Aid in the next tier.

Power Throw I will take in teamfight heavy maps such as Cursed Hollow if (1) our team has less dive than their team AND (2) their team does not have Nova. The extra range is good for poking bushes and knocking people off cap points.
T3.

Secret Weapon unless you need First Aid against enemy Nova, Tyrael, Illidan. I don't think any other talents are viable in this tier. Battle Momentum is also an option if you are building Charged Up / Static Shield.

T4.

Mighty Gust is usually better for me, but both are pretty good ults. If you take Hinterland Blast, use it during the team fight in preference to during cleanup.

T5.

Giant Killer or Static Shield. GK is not buffed by Secret Weapon. You will know which one to take based on enemy team comp. Tank heavy comps get GK, assassin heavy comps gets Static Shields.

Crippling Hammer is pocket pick for certain comps that don't have any good slows. Very rare pick, but don't forget it's there.

T6.

Hammer Time. Without a doubt one of his best talents. Nothing else comes close in this tier. Really, anyone who says otherwise is just wrong. Stuns win the game.

T7.

Nexus Frenzy, Epic Mount, and Wind Tunnel are all viable. I think Wind Tunnel is underrated. Basically it casts your mighty gust 5 times in a row, once when you press R, and four more times in the next 4 seconds. Wind Tunnel is best used to smoosh the enemy team into a wall, where they will be stuck and heavily slowed for 5 seconds. A very very underrated ability.

Epic Mount is the answer if the enemy team has Abathur or other evasive split-pushing specialists. Great for murky hunting as well.

Nexus Frenzy is all around good and gets better with SM and Secret Weapon.