Treya from KOTOR 2. She didn't want to conquer the galaxy, she wanted to destroy the Force itself (which would theoretically extinguish all life in the universe) and came close to pulling it off...
Big Brother being the runner-up. For several reasons: first, he was commentary on real people and events. Second, he is a sneaky ************; everyone is too paranoid to oppose him. Third, he forces potential rebels to constantly question WHY they want to oppose him. Finally, he might not even exist. Yes, that's right, he's most likely a group rather than any one person. That makes him scarier because he's an IDEAL, and those don't go away, they don't die quietly in the night. As long as the people believe in him, he's immortal, and without the assistance of pathetic magic.