For real evil, you gotta go with someone totally unrepentant and amoral, someone who doesn't waste time trying to explain their problems away with cheesy justifications.
I got five favorites.
Kefka - Not only was he cartoonishly evil, but he got results when he actually took out most of the world.
Lavos - I contend that Lavos might be the most creative concept for a villain ever. Billion year old space locust/starship that you can only fight with time travel and the collected weapons of at least three different civilizations.
Sargeras - Lots of villains start good and turn to evil. But it's a rare thing where that villain doesn't waste time trying to justify that slide, even if its only internally. If you have principles, but abandon those principles for pure nihilism, did you ever have any principles to begin with?
GLADoS - Jigsaw's got NOTHING on this maniac's traps. The lying, deceptiveness, and dishonesty never stops. The apologies for the deceptions are the only things that sound like obvious lies, which is the ultimate rebuke. Is the whole thing a massive training course/indoctrination session? And is there a person behind that curtain pulling the strings, cleaning the traps, baking the cakes?
Silent Hill - Normally Evil comes from a being, even if it is a mythical and invisible being. But its a rare work that makes a piece of geography so evil that it actually tears away from the rest of the world.