Knights of the Old Republic II, 2004
Surik: "Kreia, what are you—are you a Jedi, or a Sith?"
Kreia: "Does it matter? Of course it does, such titles allow you to break the galaxy into light and dark, categorize it. Perhaps I am neither, and I hold both as what they are, pieces of a whole. Know that I am your teacher, and that is enough."
Surik: "Then what were you?"
Kreia: "What do you wish to hear? That I once believed in the code of the Jedi? That I felt the call of the Sith, that perhaps, once, I held the galaxy by its throat? That for every good deed I did, I brought equal harm upon the galaxy? That perhaps what the greatest of the Sith Lords knew of evil, they learned from me? What would it matter now? There is only so much comfort in knowing such things, and it is not who I am now."
―Meetra Surik, questioning Kreia
What makes Kreia so compelling is that she always has a lesson to offer, and she's a harsh teacher that makes you question every decision you make. For example, you land on Nar Shadaa in the slums, and a beggar approaches you asking for money. If you refuse, Kreia reprimands you for being so close fisted; if you give him money, she claims that you've made him weak by denying him the opportunity to work for himself. Prior to the events of the game she was a Dark Lord of the Sith but was exiled by her pupils, and having been shunned by both Jedi and Sith, she turned her hatred to the very thing that gave both power: the Force. She then attempts to use the Jedi Exile (your character) to destroy both Jedi and Sith, and in the end you face her as your final test.
Between her cryptic remarks, unsettling nature, and apparent treachery, Kreia makes for one awesome, perfectly done character. While it comes as no surprise that she ends up being an antagonist, I'm not so sure she was in the wrong. And that is enough for me to be put her at the top of my list.