Lord voldey, obviously more powerful than magneto, vader, and the joker COMBINED, is the best choice. Not to mention his level of insanity is off the charts, I mean, he didn't even hesitate to sacrifice hundreds of his followers in order to breech the walls of Hogwarts.
Not once in his lifetime has he performed a good deed, he is responsible for thousands+ wizard deaths, including Harry's innocent parents. Sure, he was defeated with "love", just like any other Carebear antagonist, but at least he wasn't killed by some middle aged guy in spandex. In his earlier days of the HP series, he enjoyed his time as a creepy baby fetus living under some guy's turban, and EVEN THEN he was capable of plotting murderous schemes against our favorite boy wizard!