I like peace-loving species. Especially when they kick the **** out of evil ones. =] And plus, I don't know why, but I'm horrible at the game when not playing Torian... Its strange, so I'd count them as my top favorite.
The Thalan are second for me, because I always thought insectoids were cool. Like some mega-evolved super ants. Ants ftw.

The Korx, too, are cool and the only evil race I like. Like a race of Boba Fetts.
I have a problem with the Yor, though. How come you need farms to sustain a robotic population? And if they're robots shouldn't they have zero morale rating, and be able to colonize any enviorment (well, except aquatic)? .... And what the hell do they need money in taxes for...?