Okay, a lot of good suggestions above. Here's a few more that I consider to be all-time classics:
Ares : great fighting manga. The art is wierd but it grows on you, a must-read overall.
Lucifer and the Biscuit Hammer : hard manga to describe, but once again, just a great work overall. It's a tight story without filler, strong and unpredictable characters.
Onani Master Kurosawa : This is my favorite manga. It's perfect length, with a simple message kept pure, and the best "growing up story" I've come across in any medium (books, movies, anything). Just need to get past the "unique" concept .
ID : its subtitle says it all: "the greatest fusion fantasy". It's a great mix of all sorts of fantasy concepts. I'm adding this one even though it's not complete and nobody is translating it at the moment, because even incomplete it's 1000% worth reading.
I'm trying to keep this list short and sweet. If you like these and want to chat about them or want other manga, feel free to msg me.