Paper Mario TYD, Metroid Prime 1, Baten Kaitos, F-zero, Star Fox adventures is pretty good, not a masterpiece but i really liked it, maybe wind Waker and Soul Calibur II, man GC had a lot of fantastic games.

Harvest Moon: A Wonderful Life
I-Ninja

They were also on the PS2 but they were still amazing.

Luigi's Mansion. That was an amazing Launch Gem.

Both Baten Kaitos games

Tales of Symphonia

Viewtiful Joe 1 and 2

Shadow the Hedgehog. I have no idea why everyone hated it. The only problem I had with it was that it was too short.

Metroid Prime 1 and 2

Resident evil 4 and 0

Kirby air ride!!!!

A lot here has already been said, my final piece of advice: Ask Metal Jesus.

Eternal Darkness, F-Zero GX, Metroid Prime, Ikaruga (Dreamcast port), Resident Evil 4, Super Mario Sunshine... Man, do I love the GCN. Don't tell anyone, but "Double Dash" is my favorite Mario Kart game.

F-Zero - Tough as nails but really great racer... terrible voice acting though...
Eternal Darkness - Quasi horror/action game, aged a bit but still really cool
Rogue Squadron 2 - Best in its series and best of its genre
Baten Kaitos 1 and 2 - Yet to play the second one but a full on JRPG with a fun battle system... again terrible voice acting
Fire Emblem Path of Radiance - Really the first Fire Emblem game I liked, and still one of the best in the series
Gotcha Force - A really fun arcadey mech shooter like Custom Robo or Virtual On BUT YOU CAN PLAY AS COCKROACHES IF YOU WISH!!! .... Cheesy story and bad voicing again ....
Geist - Imo an underrated FPS that blends standard combat with a possession mechanic that makes for some nice puzzles
Lost Kindgoms 2 - Didnt really like the first one but the second was a great action card game
Paper Mario Thousand Year Door - Easily best in the series and a really great turn based RPG.
Soul Caliber 2 - Best in the series and best version
Bloody Roar Primal Fury - Another great fighting game but can get really hard