The title is pretty much self explanatory, what was the comic book (or graphic novel) that made you feel dirty after you were done reading it?
For me, it was this:
The things The Joker does in that book are so demented and cruel, it showed just how unpredictable and sick The Joker can be, and how horrible it would be to hang out with him (Or to live in Gotham for that matter). I just love that book.
Alan Moore's From Hell was pretty dark. I mean you can't get much darker than Jack The Ripper can you? Well written to say the least. It was the first graphic novel i read that didn't involve super heroes. I can only describe it as a train, slow to gain speed but once it get's going there's little that can stop it.