Ocarina of Time.
Although in all honesty, it would probably have to be Super Smash Bros Melee, because I play it almost every day at Uni, so it gets the most use out of my gamecube. Super Mario Bros. (SNES) is great fun as well. Ooo, and Resident Evil 4 is great too. Arrg, too many to make a real decision. I'll play anything that's good