SM: StormRaven: so much firepower on one flyer.
IG: Stormtroopers: i love precise and efficient drops.
Tau: Crisis: hit and run tactics are fun.
Necrons: Imhotek- Actually I really hate him, but it's because I have to play against necrons.
Sisters: Never seen them in action, but I like Arco Flagellants: they illustrate the cruelty of 40K so well.
Chaos: Noise Marines- Being a Slaaneshi fanboy: I couldn't pick anything else. ^^
Demons: Daemonettes (see above).
Eldar: Warp Spiders.
Dark Eldar: Never played against them and I don't have the slightest idea of what they have.
Blood Angels: I've heard they have a unit that can charge after deepstriking, so i'd go for that.
Orks: Weird boy. His complete randomness is very fun. :D
Dark Angels: Gotta love their bikes. <3
Tyranids: Genestealers. Everyone says they suck, but if they manage to get into close combat, they hurt a lot.
The Emperor protects.