I've said it before, and I'll say it again: Harley Quinn. Being a part of the Batman universe is an immediate 3,672,405,248 point bonus in my book, but there's more. We have a mild neurotic with occasional flashes to sheer insanity, sleeping with the Joker (probably), sleeping with Poison Ivy (probably*), and more that I'm forgetting moment (probably), all boiled down to a perky blonde package that's surprisingly acrobatic for being a former clinical psychiatrist, which is another point: the flimsy yet ironic backstory. A character that proved popular enough in her one-shot TV role to be brought back time and again, popular enough still to be added to the comic canon (a rare feat indeed), and popular enough that when noted director Kevin Smith's wife gave birth to their daughter in 1999, there was only one name for her. What more could one want from a supervillian?
* Go ahead, look it up, I'll wait.