If it was ever about the graphics, the PS2 would have died long ago, instead of being the top runner. It has always been about the games. The graphics come into play as adding feeling to the game. Ofcourse there are idiots who think graphics make the game (NFS:U players), I would much rather play a racing game like GT4, rather than a game like Rad Racer, as far as feeling of the game goes. If graphics were nothing, we'd still be playing with 4-bit graphics.
With that said, I don't believe PS2 is better than GCN, just different. Nintendo's target audience is children. Therefor, they make childish games, where fun is the only element. Adult oriented games don't have that trait, because after a short while, it would become boring and stupid. Would GTA be a top seller if it comprised primarily of jacking cars and shooting cops? I think not.
Anyway, my current favorite game for the PS2 is Capcom Vs SNK 2. Graphics don't play a big role in that game. At all.