Gantz

Gantz is my favourite manga. The series is written and illustrated by Hiroya Oku. The story revolves over Kei Kurono and his old friend Masaru Kato, who die in a train accident and become part of a semi-posthumus game in which they and several other recently deceased people are forced to hut down and kill aliens or monsters of some sort with a handful of futuristic armor and weapons.

The manga series has well over 32 volumes and a 26 Episode anime adaption. They also made live action movies about the manga. So it's safe to say the whole concept is interesting and deserves to be read.

Personally I like it because it's a mature manga with none of the shounen elements in it except the epic ones. It displays human nature and the will to survive whatever it takes. While reading it you might find yourself getting shivers because the amount of epicness is overwhelming.