While I do enjoy reading Endo's Eden, I think his one-shots are actually much better. Check out his
Tanpenshuu.
On to other manga, I suggest:
Me & the Devil Blues
Nijigahara Holograph (or anything else by Inio Asano, seriously)
Freesia (love Matsumoto's one-shots as well, though I do admit they may be a bit too grotesque for some people)
Helter Skelter and
River's Edge
The Yellow Book
And before I forget, raging seconds for james0246's reccomendations, particularly Yoshihiro Tatsumi's The Pushman and Other STories and Abandon the Old in Tokyo.