Consider this. Throughout the game, you fight Kefka a few times, and Ultros even more times, I think. By the end of the game, Kefka is dead, and Ultros is living the high life at the colleseum.

What does that say?
That Terra fried him one time too many with her fire spells. I like Ultros, but Kefka's got a bunch of soldiers protecting him and has magic to send Ultros up the river. My vote's for Kefka.