I have always been a big fan of Kefka's character. Almost, if not all, of the other bosses in Final Fantasy games have some sort of mystical factor about them. Kefka was just insane because he volunteered for an experiment gone horribly wrong.

X-Death never really did it for me. It has been awhile since I played Final Fantasy V, and I want to play it again, but from what I remember, he was just a really bad end boss.