My 3 favorite, in no particular order:
Kreia, probably the best voice acting, even if the voice itself wasn't that impressive.
Darth Sion. The best (worst?) sounding Sith Lord since Lord Vader himself. And he ain't using no stinkin' breath masks!
The HKs. Mocking Query: Coorta, Coorta? Are you dead yet? :D
Yeah. The voice actor played Master Uthar in KotOR. A Sith master in K1, a Jedi master in K2, and both pretty damn well played. That says something about the actor.