Overrated: I would have to say that a fairly overrated game would be Goat Simulator. I myself enjoy funny and intense glitches, but part of what makes them so funny is the fact that it was not intentional, and was out of place. All of the "funny glitches" in Goat Simulator are intentional, and, by definition, are not glitches. If you want a similar experience, just play any AAA game before the Day-One patches...
Underrated: Battletoads... Gamestop doesn't even have it.