hey I'm glad to see the senine getting some love in the forums =D
I'm more than happy to give my 2 cents:
-Bartender: the story focuses on the problems and lives of people who enter a bar to drink their sorrows away, with the man known as the glass of god, apart from great and simple stories of every day adulthood, divorces, corporation politics, cancer, death and the general problems one faces in adult life, you get some interesting backstories about cocktails and their origins.
20th century boys and its sequel 21th century boys. Mystery,conspiracy and thriller this is considered a masterpiece of storytelling that sadly has never had an anime adaptation. I would love to go on an on about the plots characters and story but that would give away everything so I will just leave you with this friend of my.
Old boy. A man has been kidnapped and left to rot for years in one room with only a television and 3 meals a day. After 10 years he's let go and he goes out to the world looking for 1 thing, revenge from the bastard that imprisoned him
was also adapted into a movie with jackie chan as the star =D
now for the quick fire bloody ones that everybody knows or should know:
-Berserk. Blood and tears made this story and by god will it make you cry.
-Hellsing. you all know mister Alucard and if not, fix that this instance.
-Sun-ken Rock. fights and mafia mixed with good old weird japanese sexuality and philosophy on leadership. Not really adult in it's tone but very adult in it's content.
-Jisatsutou. and island inhabited by suicidal people and their quest to survive.
those are all the ones I can name that might be interesting to read. have fun =D