So many great games on GameCube that my friends and I still play today.
I haven't seen anyone here mention Chibi Robo, a delightfully cute story with a gameplay loop that you dont really see anymore, but I think it holds up well.
Mario Party 6 and 7 are absolute classics, and my friend group plays both regularly.
Mario Cart Double Dash and SSBM are maybe the best regarded games of this generation.
Pikmin is a great challenge even today and doesnt show a single sign of it's age.
Super Mario Sunshine is surprisingly hard. I've picked it up probably a dozen times and never finished it.
Even some of the afterthought franchise games are pretty good. My partner and I played through Shrek 2 recently, and it's surprisingly fun, maybe even more so because the plot is absolutely unhinged.