Most Interesting / Strongest Final Fantasy main antagonist?

I chose Kefka, not for strongest, but most interesting. He's human like everyone else, but has gone completely psychotic. He betrayed his emperor, stole powers from gods, enslaved and destroyed an entire race (espers), ruined an entire planet whilst ruling with an iron fist, had an ongoing theme song that made its way to his final battle, and the final battle with him was also an allegory to his thinking process (1st tier = hell, 2nd tier = earth, 3rd tier = heaven, and Kefka above them all).

Strongest would have to be Ultimecia in my eyes. First FF final boss that gave me problems. XD