I start the series when I was still in the Middle School year, but I like GANTZ by Oku Hiroya. Basic plot point of the Manga is about some people who have died somehow returned back to life and they found out that they were now in the middle of 'hunting' game where they need to hunt Alien to keep surviving, it doesn't really help the series is filled with Gore, Sexual Reference, Fanservice, and straight up mindless action while trying to become a deep series where every human beings are actually trashy bastard. The most latter part doesn't really hold true though because there are some decent people among the cast of GANTZ.
It was your straight-up all galore and edgy that you could expect from a Seinen/Adult Manga.
Ironically, around the time I start my High School year, I get caught up with another Manga created by the same author, it was INUYASHIKI and the plot was about an old man who life depressing life suddenly get the big changes of his life while having catharsis moment from all of the pressure and his nearing dying moment. Something that seems to be Alien Spaceship whatnot fall down at him and recreates his whole being as Cyborg, the old man then proceed to learn about his power and determined to become someone who would help people as much as he can, but ironically, he's not alone when the Alien Spaceship crash because there's young man there at the same moment and become same cyborg as the old man and he decide to use his newfound power to murder people because it's helps him feeling 'human'.
The feels more or less same with GANTZ, but this one shows off a bit of Superhero-esque story from it. I can't really recommend any of them to your usual Manga reader because of the content, but I am enjoying my time while reading them while most people just say that both of the series are trash.