God Tier is a term that gets tossed around a lot these days, but there's no real definition for it. It basically just means, which manga an individual believes to be the greatest manga concievable. Perfect story, characters, artwork, often times deeper themes. Just excellent all around.
I was just thinking about it because I thought about what manga are better than One Piece. OP is my personal favorite manga but I will admit there are probably better series than it, from an actual writing standpoint I just haven't read any of them. These series are, among others, Nausicaa, Lone Wolf and Cub, Phoenix, Monster, Akira, Vagabond and Berserk. I actually have read most of that last one and it is as good as One Piece, if not better.
Anyway, I just wondered what series people on this forum considered God Tier and why. I don't think I can really post one of these lists because I probably haven't read enough manga. As can be seen above, manga that are generally considered god tier for sure are usually Seinen (except for Nausicaa and Phoenix but those are non-traditional Shonen, from what I can tell). But don't be afraid to list a Shonen you consider top notch. I would like to know why the 7 manga I listed are considered so great, since I would like to read them at some point in the future.
Anyway have fun gushing about your favorite manga, or at least the ones that personify the best the medium is capable of.