Sentenal, as much as I agree that Magus can be fiendishly diabolical when he needs to be, he is by definition and anti-hero, not a villain. He is both a protagonist and an antagonist, but neither one exclusively.
I voted Zeal, because... well, ya can't get much more evil than being willing to endanger the entire world, including your own loyal followers and children, to attain immortality. She may have been touched in the head by Lavos, but she still made the choice and, for the icing on the cake, enslaved the Earthbound population to build the Ocean Palace. EBIL![/b]