Yeah the G.U. games are literal 'volumes' more than they are sequels. If you want to make it one game, I guess you can just take "Last Recode", the rerelease that includes all volumes in one.

I must admit I haven't played Atelier Iris, but .hack//G.U. is one of those games that I'd be perfectly happy to see win the entire competition, so I'll throw my vote in still. Such amazing games, and Haseo is easily one of the best protagonists in any RPG.