I voted for Kreia, one of the most fascinating fictional characters I've come across.
It's not the methods and machinatinations so much though as it is the reasons. She was plagued by delusions of her own self-importance (and how not...as a "Sith Lord" and Revan's mentor?) and yet her belief that the Jedi had failed in their position of arbiters of right and wrong seems justified given their actions in KotOR and TSL (indeed, one can even draw connections with the failings of the movie-era Council as well, and draw out the contrasts with Luke's New Jedi Order from EU).
As for the others:
Bastila - her preachiness is annoying, and yet it suits her character, along with her righteous indignation when you step out of line. The idea that she's your mentor and conscience somewhat fails the first time through, but on subsequent playthroughs it works brilliantly IMO, because, knowing you are Revan, you appreciate how futile her task is (indeed, the futility of the Jedi Council using you as a pawn at all) given both your power and ability to shape your own destiny. Plus she's teh hawtness and the accent is one to make men's hearts melt.
Juhani - not a fan of the look, but it's the voice that really turns me off to her character. The slobbery trilled Rs are suppsed to call to mind purring or something, but it just sounds horrible. Her story is pretty cool though, though I wish there was a bit more introspection regarding her confrontation with the genocidal maniac slaver.
Mission - meh. she's fun sometimes and can offer some pretty entertaining/funny commentary, but can be way annoying as well.
Handmaiden - cool character with a nice story arc. One of my complaints with TSL compared to KotOR is that the characters don't tend to react to events outside of their own personal arcs, and Handmaiden is no different. In the end it leaves them feeling a bit flat. I won't say I find her attractive exactly, but the character model seems to suit melee combat really well to my eyes.
Visas - shame the best voice in the series gets so few lines. I wish we got to know her better, and yet the way she is withdrawn fits someone who has been the victim of abuse at the hands of someone as domineering as Nihilus appears to have been. That IMO makes her character pretty genuine, just not as developed as the others resulting from lack of background and emotional insight.
Mira - best-looking 'chick' in the series. I quite enjoy her personality too, though in some ways it is so independent that it just doesn't feel right including her in my party.