I'll agree with what Kishi said. Vagabond is quality, man.

And I'll add Lament of the Lamb. Although the only bad thing about reading that was that everything after it I hated. Until Vagabond. :D You better not mind depressing, though.

And I don't know if the manga is anything like the anime, but the Gantz anime was terrible. Just overridden with cliches, I felt.