I would give an obscure answer, or say something sarcastic like Calender Man, but like the many people here I will say the most obvious: The Joker. Why? Because it's like he's TAILORED to suit every single one of the things people like nowadays, he's insane, murderous, funny, cool-looking, well designed, well-rounded, has a good backstory written fully by ALAN FUCKING MOORE in one of the best Batman graphic novels, The Killing Joke. And on top of all that, he's INTERESTING. Joker actually FEELS insane, psychotic and creepy, from his violent mood-swings, personality changes and obvious schizophrenia, to his taste in clothing. He is, for all intensive purposes, completely and utterly awesome.
I know people vote against The Joker sometimes because the hordes of sheep into comics will say The Joker, but to me it's the one totally mainstream character that I LIKE. The Heath Ledger Joker didn't even sway me, it was a good portrayal, but in the end I preferred the Post-Crisis comics Joker. Insane, murderous, with a twisted sense of humour and a snazzy dress sense.
If only he looked like this [http://i337.photobucket.com/albums/n370/ParallelStreaks/hon01_4542c3ddf015d-t.jpg] in The Dark Knight. Would've been awesome.
..If I had to choose another, prolly Prometheus.