Boku Girl
This is not yaoi. I repeat, this is not yaoi. This is not Shounen Ai either. This is just an ecchi seinen work. And a great one, at that. In the hall of fame for Ecchi Seinen and gender bender works.
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.
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.
Okusama Wa Joshi Kousei
Arigatou
7/10, but will round up: There were absurd parts, and character development was not the strong part. However, the manga did what it was supposed to do - it made me uncomfortable. The story is a tragedy, but it does what it should do, and ultimately ends not in an unrealistic, overly wholesome way, but there is closure.
Gokujou Drops