There are a lot of lost treasures among the GameCube library out there. It really depends on what kinds of games you guys like.

I think every game that has been mentioned so far is worth your money. Most of them are published by Nintendo, after all. Here are some more that I personally own or would recommend:

Viewtiful Joe (and/or VJ2)
Eternal Darkness
Beyond Good and Evil (if you don't have it for another console)
Prince of Persia (if you don't have it for another console)
Rogue Squadron III: Rebel Strike (if you're at all into Star Wars or flight games)
RESIDENT EVIL 4
Animal Crossing
Ikaruga
Fire Emblem: Path of Radiance
F-Zero GX (Hard as , though)
Skies of Arcadia Legends (A little dated now, but still fun)

Make sure you have a couple wavebirds, too. Not many things are more lame than having a brand new wireless console that you use old wired technology with.