The guy destroyed large places with extremely powerful magic on a whim. I thought he was fantastic as an all-powerful enemy who genuinely looked to destroy the world, rather than one who just wanted to take it over or even just to take over part of it, or enslave people... this guy was out to obliterate and he actually did cause severe damage to the world. Compare that to many of the later bosses who threaten to do things but never really get there... well, I would say that Exdeath is arguably the most "successful" and the most evil of all the FF bosses. And that's saying something. I don't know much about some of the main enemies (III, IV) but I imagine that Exdeath and Kefka would be the big debate over who was the most successful with what they set out to do.