It has to be Kefka without any doubt. While others were evil too, Kefka was genicidal. His only plan was to kill. He poisoned the whole nation of Doma, he killed Espers bye the hundreds just to become more powerful, and the only one who attempted to stand up to him was murdered by him RIP Leo. The guy wasn't evil per say, he was just extreamly powerful, and crazier than a 50 year old who bought an engagement ring for Mily Cyrus with his wages of his janitorial job. So ya Kefka bye a landslide.