Boku Wa Mari No Naka
This was beautiful. This is definitely one of my favorite works. And Oshimi Shuuzou is actually one of my favorite mangakas. Also read Aku no Hana, if you like dark, twisted manga.
Death Note
Ahh, Death Note. One of the most famous mangas ever created. But if you actually do read it, it gets tiring. Could have definitely been a shorter work. But, this had an amazing concept.
Death Note Oneshot
Describes the end of Death Note without any meaningful addition to the work itself.
Dragon Head
Now this was very good. Could it have been better? Sure. But is it worth reading? You bet your yaoi-infested mind it is.
Gantz
This is one of my favorites. Dark, twisted, sexy, and completely crude - just the way I like a seinen work. A long one, but def worth the read.
Helter Skelter
This is very dark, and so, so good. You never see a dark josei work. But this one is excellent. Please read it.
Himegoto - Juukyuusai no Seifuku
Very dramatic. But an insightful exploration of different kinds of people, if that makes sense.
Holyland
I will never forget that summer. The summer when I first read this manga.
Boku Dake Ga Inai Machi