Orks. WHHAAAAAAAAAAGGGHHHHHH!!!!!!
For a few reasons:
1. The Orky Mob Bonus. wiki.reliccommunity.com/Orky_Mob_Bonus!
2. Their huge inf cap helps with ^ and overwhelming opponents.
3. Starting out, they may be weak, but fully upgraded at the Pile o' Gunz, even a Slugga Boy squad can decimate an entire SM or IG base.
4. Their backstory, and how they reproduce, as well as their attitude and mannerisms.
5. Their vehicles are pretty fast and own pretty hard. Wartraks are pretty cheap, quick to build, do nice damage, and are fast as hell. Not to mention their relic unit is a giant mastodon with a shock gun on it's shoulders, and can carry a lot of infantry, AND is tougher than the SM's Land Raider when it closes to melee.
Eldar are too weak, SM's don't have enough variety or morale bonuses, Chaos is a little too slow (takes too long to build stuff), Necron vehicles suck hard and are too slow, DE are a bit too weak, and underpowered, Tau are glass cannons, IG infantry are far too weak, and SoB are just lame, better to play as the SM or IG, and still do better than the SoB.