I can only really recomend the games I have, that are exceptionally fun. Some of them have been mentioned but to enforce the point

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Mario Kart: Double Dash - If you've played the others you know what to expect but you know an updates always fun. I honestly didn't find the "double" part that much of a factor in the game but then again I wasn't great at it XD Good multiplayer fun though.
Donkey Konga!!! :

I LOVE this game, I honestly think its so fun. There's a good mix of songs, traditional Nintendo stuff and charty stuff, and a sequel that provides even mroe songs but imo when you play multiplayer this game goes beyond the songs, its jsut so funny and addictive, its provided me plenty of laughs! There's a bunch of throwaway extras that you can purchase to keep you amused but the core of this game is exactly what you, and tis what you get; like DDr etc. but far more fun with the bongos imo.
Metroid Prime: A very smooth enjoyable game, I'm not very good at it, find it a bit ahrd to ge tinto that zone of fps/action that it has going on but after an hour or so I can get lost in it.
Starfox Adventures : lmao ok, its true, this game rips Zelda off in many ways. But hey; is that necessarily a bad thing? There aren't that many new Zelda gamers flying about on the GC anyway and if you liked Starfox, you ge tto revisit old characters. Good fun.
LoZ: The Wind Waker : I don't know where I am on this. Definetely worth picking up oppose to not buying, but its very gameplay driven; tbh I expected a little more from the story, but hey, thats not really LoZ's point so its a bit of a pointless criticism. Once you get warp the world really opens up from the slow boat sections and there's plenty to do and explore. Lovely design as well imo.
Tales of Symphonia : Excellent! By far the best rpg on GC without a doubt imo. Although it was released on ps2 in japan its exclusive to GC here, and is very much worth buying the console for alone, given the pricing of the GC these days. I finished the game in about a week but I have top admit I was up 24/7 addicted to it, so you should get a decent length out of it otherwise. Very enjoyable plot imo only wish it didnt overshadow its twists so much, I would have been genuinely suprised many times if they had not many each twist so obviously jsut prior to it happening. Addictive battlesystem too. A must!
Super Monkey Ball: Again I'd say this is really fun and is basically laods of little mini games and puzzles loooped together while travelling as a monkey. in a ball. haha. Really picks up with multiplayer but it keeps me pre-occupied otherwise too!
Resident Evil 4: I haven't played the PS2 version so I can only talk about this on its own merits; which means I can only say good things. Imo the best installment of the series so far after a few years not doing much. Very good and again, possibly worth purchasing the console for were it exclusive.
Thats all I have for my GC

plenty more I want, but I'd recomend all of those