Pac Man Vs. gets my vote!
No one has really heard of the game and when I pull it out at parties, everyone loves it. Lots of strategy with this game too! The only downfall is that you have to play with 4 players. The AI is pretty stupid. It came bundled with R: Racing Evolution, Pac-Man World 2, and I Ninja. It may be hard to find now, not really sure, but I haven't seen it @ gamestores for a while.
When I first got my GameCube, one great game that many people have forgotten about is XG3 (Extreme-G 3). I can not believe how beautiful the visuals are and the speed of the game. It was a blast to play and totally blew XG2 (Extreme-G 2, n64) out of the water. Totally worth a buy if you can find it!
Some games that got some good review scores on GameCube that many people haven't heard about are Dr. Muto, Aggressive Inline, Disney Sports Soccer, and Scaler