I thought that choosing my favorite villain from a movie wold be a lot harder that it actually was... I'm sure there were a lot of characters that I even forgot to consider. I started to think through my choices. Voldemort, from the Harry Potter series, is like this force of evil, but somehow he's not that unstoppable (Harry proves that again and again...) and secondly he's very predictable. Hector Barbossa, the pirate constantly stealing Jack's Black Pearl in the Pirates of the Caribbean-movies, is both cool and funny, but I wouldn't consider him in the "most evil-class". He conducts Will and Elizabeth's wedding after all! Davy Jones, the tentacley half man half sea god from Dead Man's Chest, is closer in terms of no compassion. He kills Will! But he's lovesick, which isn't very flattering for a villain.
But the winner of them all is just pure, illogic and twisted evil. No emotions, just for fun. Specialize in ways to destroy people. He's a joke. A twisted, sick joke.
have you guessed it?
He's the Joker!
Surprise! I'm a huge Batman-fan, and Joker is his arch enemy number one. And when it comes to portraying him on-screen, I think the entire world agree that there's none over or beside Heath Ledger. It was his last big film, and it wan him an Oscar. No wonder, actually. It's truly impossible to describe the Joker to you if you haven't seen "The Dark Knight". And if you have, you don't need an explanation! EVIL!
Rip. Heath Ledger, and your performance will live on for centuries as a true icon by a true genius.
Check out Elisabeth's answer at Drops Of Art!