I've always thought that Kefka was a somewhat shallow character, he's got some memorable lines, okay (self-help booklet, anyone?), but he shows no interesting motives, he only wants to destroy everything. He's a good character (he destroys the world, after all), just not great.
I choose Sephiroth. Better developed character, perhaps not as hateable, but memorable nonetheless.
Though, in a fight, Luca Blight owns both. After all, Kefka was taken down by a mere 14 characters, Sephiroth by 9. Luca needed a complot against him, a battle against 18 characters, an army of archers and an one-to-one duel to go down








