If you still play GBA games, then I definitely recommend getting a Gameboy Player for the Gamecube. It's a nice addition to the console, blends really well with all colors, and you can play Gameboy games and record them with a capture card. Of course, there's always emulators, but what's better than recording the real thing?
I also recommend getting Zelda Four Swords Adventures, it's a pretty good game. If you don't have it for Wii or Wii U, I would definitely get Twilight Princess. (I have it for Gamecube, and I'm STILL gonna get it for Wii U). There is also that Zelda Master Collection if you enjoy getting collections.
Warioware Mega Micro Games was also a good party game too. Definitely put Mario Party 4, 5, 6, and 7 on your list, because the Mario Party games are always fun to play.
If you enjoy Sonic games, there is also Sonic Adventure Director's Cut, Sonic Adventure 2 Battle, Sonic Heroes, Shadow The Hedgehog, and also Sonic Mega Collection and Sonic Gems Collection, which both give you a whole set of games.
In terms of Pokemon, Colosseum and XD are good, but there is also Pokemon Channel to play with Pikachu!
And you CANNOT start a party without Donkey Kong! There are plenty of Donkey Kong games to choose from, such as Donkey Konga, Donkey Konga 2, and Donkey Kong Jungle Beat. Great games to play with bongos.
Stepping out of Nintendo boundaries, there are also games like Beyond Good and Evil, which is a pretty fun game. I played it for Gamecube, and have it for Xbox One (Backwards Compatibility from 360).