Any good Gamecube gems you know?

Eternal Darkness.
Resident Evil 4.
Super Smash Bros.
Resident Evil Remake

Baten Kaitos for sure. One of the best games i played.

Tales of Symphonia was my favorite back in the day. Its probably cheaper to get it on steam but the original version is actually the best because of 60 FPS.

Viewtiful Joe 1 & 2
Okami
Ikaruga (though you might as well get the steam version)
Star Wars: Rogue Leader
Time Splitters 2 & 3
Eternal Darkness: Sanity's Requiem
Fire emblem

Alright, so not the most well hidden games of all, but they're all at least quite good.

Luigi's Mansion
Pokemon Colosseum and XD Gale Of Darkeness
Kirby Air Ride

The Legend of Zelda: The Wind Waker (2003)
The Legend of Zelda: Ocarina of Time Master Quest (2003)
The Legend of Zelda: Collector's Edition (2003)
The Legend of Zelda: Four Swords Adventures (2004)
The Legend of Zelda: Twilight Princess (2006

I still have my Gamecube set up for when my brother wants to play Mario Kart: Double Dash. My other most played games were;

Pokemon Colosseum and XD: Gale of Darkness
Star Wars Rogue Leader
Godzilla: Destroy all Monsters
Fire Emblem: Path of Radiance
The Simpsons: Hit & Run

Resident Evil 4 is an obvious one, a true masterpiece. Of course you can play it on everywhere else since Capcom keeps re-releasing it on every possible system they can think of.

Wind Waker - Probably the best Zelda Game.
Paper Mario Thousand Year Door - Good solid RPG with nice action elements.
Phantasy Star Online 1 & 2 - one game package. good even without online. Splitscreen if that's your thing.
Starfox Adventures - Good adventure game
Mario Sunshine. - Underrated platformer
Metroid Prime - Another great action/shooter adventure.

Those are all uniquely Gamecube.

Killer7

Paper Mario. I've only played N64 version of it but it was magnificent, filled with great gameplay and long as hell. Try it. I suspect Gamecube version is even better.

There's a criminal lack of Super Mario Sunshine on this thread. That is my favorite Gamecube game, and is still one of my favorite games to this day.

My one and only guilty pleasure on the GameCube was Pokemon Stadium

METROID PRIME
METROID PRIME 2

It's on Ps2 as well but SSX3, THE best snowboarding game imo.

Donkey Kong Jungle Beat, it looks weird, I get it, but that's why everybody else avoided it. It's honestly really good, and while it does have a "unique" control method, it makes it waaaaay more memorable.

Battalion Wars, it's certainly a unique one, easy way to think of it would be Advance Wars if it was a tps.

Custom Robo was always my favourite game one it ^_^

Nothing new to add, but definitely agree with some other suggestions in here. I remember really enjoying:

Eternal Darkness
Animal Crossing
Super Mario Sunshine
Paper Mario
Pikmin 1 & 2

Edit: Just noticed your edit.. maybe some less famous games that were good:

Mr Driller
Ikaruga
Killer 7
P.N. 03

Skies of Arcadia, good game

Beyond Good and Evil (It's on Steam though)
Final Fantasy Crystal Chronicles (Great narrator, best played co-op)

Super monkey ball :)

Gamecube is one of my favourite consoles.

Idk about hidden gems, but ones not mentioned above:
Sonic Adventure Battle
Billy Hatcher and the Giant Egg
Tak and the Power of Juju
Bomberman Generation

Also Pikmin is one of my favourite games still, in case you haven't ever tried it.

Twin snakes, paper mario, ocarina of time remastered

Cubivore is great and obscure, but definitely not cheap on ebay.
Chibi Robo is another fun one that I don't see mentioned above.
Besides that, I second a lot of ones that have already been mentioned, as they are some of my favourites as well (FF:CC, Animal Crossing, Sunshine, Paper Mario, Custom Robo, Symphonia, Pikmins, etc.)

Pikmin
Teenage Mutant Ninja Turtles
Starfox Adventures
Pokemon Colosseum
Super Smash bros
Zelda