Even though is from the 6th gen (unless you count the PC version), Psychonauts. My god this is what you call an underrated game.
One of the best 3D platformers I have ever played, the creativity is something that's very rare to find in games and heck even movies, likeable characters, funny as hell, fun and creative gameplay, lots of collectables (new physic powers, artwork, etc) it has everything you want from a 3D platformer, Tim Schafer really did a great job on this one. If there's anything bad I can say about the game is that the controls are a little bit clunky, but I very rarely had problems with them even playing with a keyboard, so this is nitpicking more than anything else.
Sadly it came out too late in the 6th gen (the 360 was just about to come out), and the decrease in popularity of 3D platformers (aside from Mario) probably had to do with it. According to VGC, it sold only 210k between the original Xbox and PS2...that's so sad, this game deserves a lot more than that, I really hope the PC version sales are much better.
That's my choice from a sales perspective, the critics loved the game.
From a critics perspective....Donkey Kong Country: Tropical Freeze. I generally don't care about reviews, but 82 metacritic is, imo, way too low for this. It gets almost everything perfectly, the controls, the music, the visuals. My only "big" complain about it is that the game is very short, I personally don't have a problem with that because I'm a sucker for time attacks which will easily give me dozens of hours, as well as hard mode, but for someone who doesn't care about this stuff, it is a very short experience (if you only do the require levels you can even finish it in a single day), I guess Nintendo knew this beforehand because the game cost $50 instead of $60.