GANTZ is one of those animes that no matter how hard you try, you will never forget nor want to forget even if you didn't like it.
Even through all the violence and sex, the ideal of it is still intriguing to the thought. What does happen when we die? Are we given a second chance? If so, what will we have to do to get it? And since our protagonist isn't the usual prince charming, we can relate to him more since he's a person from all sides, wither is violent, perverted, or just down right evil.
Other than a great story, the art and music are amazing as well. Gives a very realistic depth to a very fictional story. Even though it is not one of my top favorite animes, I can assure it is one of the greats.
"Come see a side of the world that few look into, and realize that you will never go back."