Akagi - Need I really explain this? The sands in hell are magical sands. Epic mahjong. Ext.
Berserk - Again, should go without saying. Right now, it's turning into a bloody badass version of Shadow of the Colossus.
Bokurano - It's about a giant robot that's powered by the lives of the children who pilot it.
Buraiden Gai - Fukumoto. First chapter, the character is backed into an alley, surrounded by 100 cops, and the only way they take him down is with PHILOSOPHY.
Deadman Wonderland - Possibly one of the most cinematic manga I've ever read. It's also one of the best suspense series currently running.
Doubt - Yeah, ok, the concept is way better than the execution. But it's still fun to read, and it'll be interesting to see how it resolves.
Doujin Work - The manga by Gogi Power Hiroyuki, the man who started "a cat is fine too, "so moe I'm gonna die," "j-j-j-jam it in," "the more beautiful and pure a thing is, the more satisfying it is to corrupt it," "it's shit, right?," and so on. And it's filled with material on-par with that in terms of humor.
Elfen Lied - The anime is far superior, but the manga has a ridiculous amount of fanservice and awesome dialog.
Franken Fran - It's like a cross between Junji Ito horror manga and The Twilight Zone, except it's better than either. It also has one of my favorite female characters ever.
Gantz - It's similar to Code Geass, in that it's pure ridiculous entertainment. That said, it's still addicting and fun as hell to read.
More justifications coming...