Malak. He almost brought about another Sith age. All Kreia does is watch the Exile, betray the Exile, and die by the Exile. Malak kidnapped Bastila, attacks Dantooine, and kept Jedi undead in a tank. Plus Malak actually used force breach on me. I had to fight him without the force. And he had moves! His Power Attack was the best attack animation in KoTOR 1.
Damn, I am a geek!
If you had said character, then definitely Kreia. She has way more lines than Malak and a better backstory.