Poison Ivy, for both fapping and non-fapping reasons.
Obviously she's really hot. And further, I like that she's an interesting take on a femme fatale character. On the one hand, she's seductive, and actively uses this to achieve her goals. On the other hand, she actually hates the entire human race, and every man she seduces she views as less than worthless. It's the femme fatale idea taken to extremes, and Batman's villains are always best when they're normal things taken too far, into pure comic book territory.
Finally, I like when they push her as some kind of metahuman. I like the metahuman members of the Rogues Gallery, her and Man-Bat and Clayface. I don't think it's "out of place" in Batman, since Batman is ultimately a very pulpy character.
