Er, Gantz.
Seriously.
Yes, Gantz has a crapload of horrible, horrible violence, but once you settle into that and realize that it's going to be pretty much a constant, you discover the pitch black, hopeless, rotten core of the story. The way the majority of humans are presented as cynical, selfish and completely oblivious has to be one of the most discouraging things ever put to paper. And, without giving too much away plotwise, deep into the story there's an existential mind-rape moment that's bound to leave you with some uncomfortable questions. But one of its darkest, and truest aspects, is the fact that, once you analyze the actions of the final antagonists in the last arc, you can't say that there's anything ethically unsound or morally repugnant about them - they're completely justifiable and natural, and the only thing that makes them terrifying is that humans are the victims. The author does try to tie it all together for a happy-ish ending, but by then you'll be so horrifically depressed that you won't care.
I've read Berserk and I didn't feel it was as dark as Gantz. Berserk at least has some kind of ticking, human heart underneath, no matter how trampled on and broken it is. When I finished Gantz, I swore to myself that I would only read fluffy stories for the next few months or so.
Battle Royale is another incredibly dark manga, but I really don't feel like getting into that one this early in the morning. If you want to forget what happiness is for a while, by all means, indulge.