Alright, because you said underrated, I have to say Beyond Good & Evil.
I just bought it a little while ago and I've been playing through it recently. I'm fairly far along and I now wonder why I never bought it in the first place. This game is really good. I think my favorite thing is the balance of the game. If you liked Wind Waker, there are similar photo based goals in the game, and they are well implemented. The setting is just futuristic enough to feel real but still alien, there is a good mix of stealth, action, and exploration involved, goals are straightforward, bonus stuff is fun, characters are likeable, graphics look good, soundtrack is great, I could go on. The only thing you may not like is that it's a pretty easy game, though I think that's good because it gives more focus towards the story and the mechanics over difficulty. I love this game and recommend anyone try it.
At this time I would also recommend NOT playing Starfox Adventures.