Some people posting non underrated games, lol. Maybe it's a better idea to make a "What Gamecube games did you enjoy?"
So I can't believe how MUCH FUN THIS GAME IS. I'm almost finished with it but it's pretty much classic mega man platforming with a wide array of weapons and an absolutely amazing soundtrack. The artstyle is really great as well. The only thing I hated were the abundance of tutorials that weren't skippable, but I think that was only in the first area.
This deserves an honourable mention as well. Overall great game that did an amazing job for a first time genre switch. gr8 OST, gr8 gameplay, gr8 artstyle, gr8 fucking everything.
I never really had the money to buy Gamecube games left and right at the time, so my library was pretty small (I'm expanding my library a lot right now though), and the only games I really had were the popular ones like F-Zero GX, Windwaker, Melee, Mario Sunshine, etc. There were only a few maybe obscure games that I bought, including the 2 above that I mentioned.