Kefka didn't actually have NO motivation or reason, in Vector you learn that Kefka was one of Cid's first experimental magic knights, but the process wasn't perfected yet and something in his mind snapped. But whatever, he's still a damn great villain, one of the most endearing ever. Also, yes, he is the only villain I can think of on record (except for the Vogons, of course) who actually DID destroy the world.
And yes, BioShock has some great villains too. Fontaine and Suchong are obvious choices (although we do see signs that Rapture was already having problems before Fontaine came along, but he definitely deserves MOST of the credit), and, like Purple Rain said, Sander Cohen hits particularly close to home because I, too, am a pretentious art-movie buff. ^_^;