In many FF games, there are many foes. And final bosses may not be the most evil. Some may argue that Rufus was more evil than Sephiroth, though I don't know how you could say that.
So who do you think of any foe from any FF game, who is the most evil?
My vote goes to Kefka.
The guy's insane. He lives only to destroy, only to kill others. He has no greater goal or dream or ambition like Sephiroth. He's not just trying to live forever like Garland/Chaos. He is just in the mood to hear everyone scream as they are dying.
Let me ask something, though. What's the deal with Zemus? What did he have against the Blue Planet. FuSoYa says that they came to the Blue Planet, found that everyone was still evolving, so they went to the moon to wait. So why did Zemus decide to control Golbez, revive the giant and destroy earth?