1)GCN - It has the variety. Theres Mario, Zelda, SFA, Metroid, Skies Of Arcadia, plus it has the almighty online RPG known as PSO.

(Basically everything that you said about GCN).
2)PS2- I chose PS2 second because most of the upcoming games that I want are RPGs...so it doesnt really have the variety IMO. But games like Suikoden III, Star Ocean 3, Xenosaga, Legaia 2, RPG Mkaer 2 and Final Fantasy XI are all worth owning. I'm not really interested in PS2's non-rpg games, because I can beat most of them when I rent them.
3)Xbox - hmm..I guess Panzer Dragoon Orta looks nice...and it has Shenmue 2...other than that I don't much of its upcoming line up because I don't pay attention to Xbox that much.
I won't count GBA.