Most over rated: Halo, Halo 2, Half-Life 2 (all those games were getting way too high scores, and none of them was actually fun IMHO)
Most under rated: Return to Castle Wolfenstein, (because some were complaining about the singleplayer not being the most fantastic, while the singleplayer was not important at all because everyone got the game for multiplayer after playing the demo) the game got scores like 8.5 and 9.0 but it should have gotten 9.9's only.
NFSU2, got a 7.4 or something at gamespot. It's really a excellent game which deserves a 8.9 at least.
Postal 2: This isnt really the best game, but I think reviewers already aproached the game like 'this is one of these games that thinks it can sell by being ultra violent so we really need to give it a extra low score'. So the game got scores like 3.0 and 4.5, while it deserved at least like 7 because it actually was very funny.