Illidan is one of my favourite heroes due to his ability to 1v1 most heroes, including harder heroes like Cho'gall & Thrall! The only heroes I have come up against that he fails to beat 1v1 are Sonya & a really really good Murky. This isn't to say that he is weak in time fights ow no no, he has a lot of damage and very good survivability through mobility and his Evasion skill, this allows him to neutralize back lines and threaten the entire enemy team with his no-resource requiring abilities.

Passive - Betrayer's Thirst
Basic Attacks heal for 20% of damage dealt and reduce your Ability cooldowns by 1 second.

Q - Dive
Cooldown: 6 seconds
Dive at the target, dealing 145 (69 + 4% per level) damage and flipping to the other side of the target.

W - Sweeping Strike
Cooldown: 8 seconds
Dash towards target point, dealing 261 (124 + 4% per level) damage to enemies along the way. If you hit an enemy, Basic Attacks do 35% more damage for 4 seconds.

E - Evasion
Cooldown: 15 seconds
Evade enemy Basic Attacks for 2 seconds.

Ultimates:

The Hunt
Cooldown: 60 seconds
Charge to target unit, dealing 550 (262 + 4% per level) damage on impact and stunning for 1 second.

Metamorphosis
Cooldown: 120 seconds
Transform into Demon Form at the target location, dealing 101 (48 + 4% per level) damage in the area. Gain 401 (191 + 4% per level) temporary max Health for each Hero hit and 20% increased Attack Speed. Lasts for 18 seconds.

Thanks to Illidans passive his abilities don't stay on cooldown very long allowing him to spam them, due to not having any type of resource to drain also this means he doesn't have to hold back, not only this but it also heals him for basic attack damage done, this is extremely powerful and allows for easy health management! His Dive ability is great for slowing fleeing enemies as it causes them to stop for a couple of very brief moments when you use it on them, this makes it extremely good for chasing people down. Sweeping Strike does moderate damage and is another form of mobility, it increases your damage which is great and allows you to dash out the way of abilities! Evasion is extremely powerful when used correctly and can allow you to take almost no damage from Merc camps or even solo towers (keeps can be trickier). Metamorphosis provides much needed area damage for team fights and gives a good chunk of survivability, not only this but it increases your total damage output through auto attack speed which helps tie in with your passive greatly! Finally The Hunt is a long range assassin ability that gives a good amount of map presence along with being a great way to deal with those pesky mages (Jaina, Kael'thas and even Abathur!).