Who is your favorite fictional villain and why?

What makes up a good villain? What type of villain do you enjoy the most or what element of said villain do you find the most convincing and/or frightening? Is he mysterious and hidden (Voldemort, Sauron), iconic and on-the-move (Darth Vadar, The Green Goblin), intelligent and psychotic (Hannibal Lecter, The Joker) or all of the above? I'm talking about the character's personality, attributes and motives, not just his costume or special powers.