I'm a big fan of Tokyo Ghoul (and sort of on a masochistic streak, it seems) and I feel like reading mangas with a similar feel to it. I want something that has layer upon layer of despair, a story in which the moment you think the main character's been f*cked up enough, there comes life/plot to prove you wrong. Something in which you see a perfectly normal, perhaps innocent character slowly but surely keel over to depravity, perhaps withouth them noticing before it's too late. I want to a manga that makes me emotionally invested in the characters only to see them lose hope, and lose hope myself.
Let me think of some examples..
Toukyou Kushu as I've mentioned before
Kimi no Knife - that sucker punch ending left me feeling empty for days
Übel Blatt - koinzell's life story, really, but also
Spoiler (mouse over to view)
that moment when I knew that Koinzell was going to swordfight Ikfes, and that he was going to lose
some of
Aki (あき)'s works, like
Elhanburg no Tenshi and
Olimpos have a subtler, more subdued kind of that feeling
thanks ^^