Star Wars Rogue Squadron II - Rogue Leader
No other launch game in history had such stunning graphics, it's still one of the best looking Gamecube games ever made. High polygon count, sharp textures, well done mip mapping and a smooth frame rate brought gameplay to a new level for this kind of game. Nobody who played Rogue Leader would want to go back to play Rogue Squadron on the N64.
As HappySqurriel said in another thread, the step in graphics from generation five to six was much more important for gameplay than the step from generation six to seven.
A game like Gears of War may look fantastic, but gameplay wise it doesn't do anything which would be impossible on a 6th generation console. You are only impressed by great graphics for so long, but once this impression wears off, it goes back to gameplay. And gameplay wise Rogue Leader would have been impossible on a weaker hardware like the N64.