Yzma! The hilariously over the top, purple-loving, un-empress. She's one of the best parts of an already super movie, and her awesomeness even translated perfectly into the spin-off show.
Runners up are Zurg and Frollo, for very similar and completely opposite reasons, respectively. I'm unsure if John Silver counts as a full-fledged villain, but I love him as a character and would gladly include him if he does.