Wow, emperor26, you read my mind - not only did I have an idea for a topic like this just yesterday, you even mentioned Parappa the Rapper, the game that I had most wanted to highlight!

I would disagree with you on one point, though; the first game was critically lauded (for the reasons you stated) and sold in quite large quantities, so I wouldn't really say it was underrated. The sequel, on the other hand, was seen as a critical and commercial disappointment, which I think is a real shame. I fully accept that it was really brief, that the gameplay was very simplistic and that as a package it was slightly lacking, but I'm willing to forgive it for most of its faults just for its personality; it had some really charming art design, a totally bizarre (if totally incoherent) plot, amusingly off-kilter dialogue, all complemented nicely by the funky mix of 2D/3D graphics. Most importantly, I thought that the songs, which a lot of people criticised, were really good, the standouts being "Toasty Buns", the burger-cooking song sung by the ghost of a bearded European burger chef, and particularly "Hair Scare", the funktastic hairdressing ditty performed by an effeminate latino octopus barber (who forms the basis of my current avatar/personal message

).
Give it a try!
Also, I did feel that a lot of Nintendo's GameCube titles, particualrly the franchise ones, were a little underappreciated; the likes of Zelda: The Wind Waker, Mario Kart: Double Dash, Super Mario Sunshine, Pikmin, Luigi's Mansion and so forth. I certainly agree that the body of games produced by and large represented a step down from their outstanding N64 output,
but I think people's disappointment about that caused them to overlook the fact that most of these games were still very good on their own merits; hence a lot of unfairly harsh review scores, and a general perception of the GameCube library as weak, which I don't really think is fair.
(Oh, and on a slightly related note - ever since playing the also quite underrated Star Fox Command on the DS, I'm quite keen to try out Namco's GC entry Star Fox Assault, which I previously had avoided due to fairly poor press coverage. Can anyone who's played it tell me if it's actually any good, or is its bad reputation justified?)