Structured Agruments? On /a/? Well, there's a first for everything. Let's see...
The current situation in which a single Alien is WTFHAX which the entire Tokio team can do nothing about is beginning to hurt the manga. Continuing to give the 100 pointer more and more transformations is dragging on more than anything else, raising questions on whether or not the author actually knows where he's taking the story.
Killing off the main hero was a bold move that's not very common in any manga. The problem being that this manga contains a revival fest similar to Dragon Ball, which makes it possible and even highly likely we'll be seeing Kei again. If we don't, then the author wasted the entire Phase 1's character development of Kei, which makes up for pretty much everthing that's good about that part. The best parts of Gantz aren't the fight scenes, it's what goes on between and around them. The fighting itself is above par at best when compared to other similar mangas. Gantz is by no means a bad manga, but to call it the best is simply an opinion, one which people who read other, similar manga will most likely not share.