Animal Crossing is my absolute favourite. I'm actually mildly annoyed that it finally got released in Europe, because that means I might have to go and buy it again, and I bet my existing village isn't compatible with another region...
Wind Waker comes second, though. It just looks amazing, and there's so much depth to it with all the side quests, islands and Nintendo Gallery. I've easily put in over 100 hours, and I've only just started collecting the Triforce Shards. It's a game I play obsessively for a bit, put down again, and come back to. (Most games I just play and put down.)
Pokemon Colosseum was a bit disappointing for me, I have to admit. I didn't realise that the "RPG" part of the game would consist almost entirely of battles, with no talking to people or collecting objects (except that gear piece). I get bored with battle after battle after battle with no relief in puzzle-solving. And it annoyed the hell out of me that I could only play as a boy - and a boy who looked ridiculous! I mean, am I the only person old enough to remember Adam Ant? He might've looked cool in 1982, but not in 2004! :shock: The girl was cuuute (apart from her orange hair and clashing pink boots), but all she gets to do is follow around Useless Lad and occasionally point out Shadow Pokemon. Whoopie do. So Not Aimed At The Girl Gamer Market.
Nobody mentioned Pikmin? the game I basically bought a GameCube for? Loooove it, so pleased Pikmin 2 is finally out here (and trying to resist the urge to buy it - as although my bank account is currently looking quite healthy I don't know how much stuff my friend who's gone to Japan is bringing back for me, so I shouldn't buy anything non-essential until I know!).