Ghost in the Shell (Movies and Animes). The movies are deep on so many levels that I spoiled myself, after watching them i went out looking for more in the same vein but nothing could have ever compared.
Black Lagoon - It's like Cowboy Bebop but a bit more over the top. Plus I just dig bad ass rude chicks with tats and dark hair

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Cowboy bebop/Samurai Champloo - they're great, so great I really dont need to say nothing about them, reputation speaks for itself.
Neon Genesis Evangelion (original) - it can get pretty dark sometimes and watching it through my teen years didn't help much
BTW Gantz anime is pretty low budget. If anything I'd advise youyou read the manga's which were soooo much better but it was dirty, i mean dirty as in x rated. Rape, murder...sometimes even at the same time.
I've got more but they're all old school as I dislike almost all new anime.