Well, now that I've had my birthday and Christmas, my GC library has expanded to:
Metroid Prime
The Legend of Zelda: The Wind Waker
Soul Calibur II
Super Monkey Ball 2
Need for Speed Underground
NCAA Football 2004
All are good games.

For those who don't hate sports games, I HIGHLY recommend NCAA Football, even if you're not into college football. It's possibly the best sports game I've ever played. There's a whole helluva lot of stuff, plain and simple.
I don't like Super Monkey Ball as much as the other games. I haven't played multiplayer/party games yet, but I will soon and they look fun. But the single player story/challenge mode is always either frustratingly difficult or boring.
Soul Calibur II, although an improvement over the original and therefore very good, is not fresh enough to rekindle your interest in SC if it's died down. I got bored of it quickly, although occasional vs. fights keep the disk from getting too dusty.
Metroid Prime is HUGE and pretty interesting, but not really that fun in my opinion (and apparently no one else's). I just can't get all that into it, although I think I'll try getting 100%.
Need for Speed Underground is great. The different modes keep the game fresh, and each one is fun in its own way. The graphics aren't as good as I was expecting them to be, but they're decent. The soundtrack is a nice comination of rock and hip-hop, and the racing itself is the best I've seen from a NFS game.
I've barely played Wind Waker at all, but it looks like a fun game.