Sorry for reviving this thread, but I thought it would be better than just making another thread, since it was only on the second page.
Either way, I think the most underrated game ever is Grabbed by the Ghoulies for the Xbox. Like another Rare (as in the company, not as in hard to find) Kameo, this game started out as a Gamecube game. I know alot of reviewers said this game was crap, but there wrong. It's actually a really fun game, and quite a challenge. Plus, it has some of Rare's ol' subliminal sexual humor (that only older people can understand) that was found in the Banjo games and such.
Another underrated game? Well, sorry for sounding like a Rare fanboy (which I actually kind of am, though), Starfox Adventures. I never got to beat it since I was stuck on that one part with Musclefoot where you had to jam the A button constantly and because I sold my Gamecube (I still own the game, however), but I thought it was pretty addicting. And while many find Tricky to be annoying, I actually like the little guy.
Last but not least, up for a tier, Galactic Pinball and Psychonauts. Galactic Pinball is just a blast to play on the VB and it easily my favorite VB game that I own so far. And Psychonauts reminded me of some of my favorite platformers. It reminded me of Banjo-Kazooie, Sonic Adventure and even Conker a little bit. It's a shame not many people played it, because it needs a sequal.
There are also a few other games I'd put on this list such as Kameo, A Boy and His Blob and Beyond Good & Evil, but I havn't play much of them. Also, I wouldn't really call Diddy Kong Racing underrated, because as fun as it is, it's (along with Goldeneye) one of the few games by Rare that doesn't get any bumrap (well, only that review from Gamespot gave it a mediocre rating. That's it, though).