Kefka.
Okay, look... Final Fantasy 6 is a good game. I enjoyed it. It's fun. But Kefka was simply born to be hated, not loved. He's amusing to a point and effective as a villain, but let's get something straight here... Kefka is a foppish clown trying desperately to be the Joker, but I think the actual Joker would do better in his position. He's an entire circus act wrapped around genocidal hate-monger.
He gets credit for re-arranging the face of the planet and stealing the power of the gods that made it, but let's put up one harsh reality about that particular notion: This act and everything that led up to it was made possible by HIS EMPEROR, an actual strategist and leader with a little horse-sense, like..."Re-arranging the face of the planet is BAD.".
He's insane, but I don't think too highly of the opinion that he's actually deep. I think the display we see is exactly what it says on the tin. He hates everyone and everything, likes killing and maiming, enjoys ultimate power, and looks very VERY flaming...except that no one would ever REALLY want to get near him on either side of the board.
And yeah, I do like Sephiroth better, but while there might be Kefka fans chomping at the bit to tell me off, I don't really worry too much about people who don't like Sephiroth. The Jenova-spawn can look after himself.