Gantz is perhaps the most knowingly controversial manga I have (and continue to) read. It's very reminisent of Battle Royale, except it has a very mysterious sci-fi slant too.
I say controversial as in entire school classes being massacred by a twisted alien or a nutcase, who after dressing up as a black man, walks down a Japanese high street shooting men, women and kids with a machine gun. It borders on being sadistic, though the author has left it up to the reader to decide what is right and what is wrong. There is no sympathy but no real pleasure either. It's sad, but Gantz is the type of extreme story that just couldn't be produced in America or the UK; it's creative freedom gone awry and I love it.