GameCube one for the newness. Although CF is the best game in my opinion, it wasn't new enough. Which is also one of the reasons it gets old for ACers a little too fast.
I think it would be pretty cool if somehow, there could be a re-released version of ACG in which we had internet connection... but it probably won't happen. It would basically be AC:CF without the city, and with all of the old features; which raises another question, why doesn't Nintendo release an update for the game through Wii Internet that would allow things like errands to come back?