Well, it depends what games you're into. ^_^ I actually like RPGs more than Adventures. So I'd settle for:
First Choice: The Legend of Zelda - The Wind Waker: Great game. Although it has very bad graphics (like cels, giving it the nickname; "Celda"), it is still a good game. Not as big as tOoT, or MM, but it is enjoyable, and some parts are challenging. The storyline is original and interesting, and the characters, dungeons, and towns are all uniquely designed.
Second Choice: Pokemon Colosseum: Pokemon games are my second favorite type of RPGs (games like Zelda coming first). Pokemon Colosseum has a few drawbacks, but good points, too. First off, Pokemon Colosseum lacks the "C-control", the ability to move the camera around to change the angle. Although it is 3-D, it is not as free as most games. The C-control lack can be very annoying sometimes. Second, the graphics are rushed. Thanks to the "wonderful" Bill Gates's X-Box, Gamecube has to rush a lot of their games these days. When looking at the shadows (and even characters and locations), there are often glitches in their appearance. The good points of this game are that the storyline is good and deep, and the protagonist a hottie. ^^
I still enjoy a nice Adventure game sometimes, though. My first and only choice is:
Super Mario Sunshine: This game is NO WHERE near as good as its "predecessor", Super Mario 64. While it had over 15 worlds to explore, SMS only has 7 (9 if you include the final Bowsa battle, and the homing point (Delfino Plaza=Peach's Castle on SM64). And they are no where near as grandly designed as SM64, either. But to compare these games is unfair. This game was rushed (thankyou, X-Box), to keep Gamecube going. It does, however, have a good storyline. Deeper than SM64, and it has more plot. It also has amazing graphics. Although they could have done better with the shadows. (Circles, except Mario.) An annoying fact about this game is all the invisible walls and fake doors. Nothing annoys me more than them. But it is an Adventure game, so such is to be expected. If you've ever played SM64 (which most people have, and if not, some of its misses will score with the new version, Super Mario 64 DS), then just think of Super Mario Sunshine as a SM64 clone (although no where near as good).