OMG!!!!!!!!!!!!!!!!!!!
First the Mongols. They are built for war man, plain and simple. Tech HBR before mathematics, it's worth it for them. If your a peaceful builder, then I agree they suck.
Ghengis: he gets military benefits all over the place. The UB directly effects his UU which I think is great (see previous threads for these arguments, I won't make them again). Double GG points. He's agressive, fast barracks and drydocks, and from combat I. Best war monger in the game.
Kublai: Agressive trait and UU/UB same as his mongol grandfather. Creative allows fast culture border popping for captured cities, what's not good about that.
Incans. The BEST in teh game IMHO.
The UU is a double edged sword early on. They can be mass produced and kill every/anything that moves out there. On the other hand, they are cheap and free (non-tech) city protectors allowing Hyuan to tech everyreligious and economic thing under the sun. Lot's of hammers saved that can be turned into wonders. He's the AI most likely to get the oracle and free metal casting (very early and cheap forges). He's financial, enough said there. If that's not enough he's got a UB that essentially turns him into an instant creative leader.