I think the general consensus here seems to be that Kefka was more insane than evil, though I'd certainly say he was a bit of both. Certainly more insane than evil, though the actions that his insanity drove him to, such as poisoning the water supply of Doma would certainly qualify as evil in my book.
I was initially going to vote for Kuja in this poll because I've always considered him amongst the most competent and evil of the villains but having read NeoCracker's analysis, I've had second thoughts. Although he certainly acted in his own interests and in an attempt to preserve what he had which is very indicative of human nature really, I think the fact that he was willing to use anybody or anything to achieve his selfish goals would count as selfish.
As it is, however, I can't think. Kefka, Kuja or Palamecia? I think the obvious solution is just put off voting for now!