In all honesty, the custom races usually own in my games, oh wait... you meant the AI.
Honestly, before the beta it was pretty much fair game, a lot of it has to do with starting positions though. I've fought from Drath, to Dregins, to Altarians in endgame scenarios. Now though, the Torians and the Thalans are usually very strong, between them and myself, the others just haven't been fairing very well... the two times I turned them off, it seemed pretty much like usual, Drengin and Arcean in one game, Drath, Terran and Yor in the other. So I edited the Torians and Thalans before turning them on again. This was from beta 1, still testing the second...