Overrated:

Gears of War. I think the cover-based shooting mechanic is interesting but it got wearisome over the course of the game, and the game's grim-n-gritty style is unintentionally hilarious.

Diablo. Click, click, click, click, click, click, click, click...

The appeal of the entire MMORPG genre completely eludes me.

Bioshock. Take System Shock 2, dumb it down in every single way except prettier graphics and sounds, laugh your way to the bank.

Half-Life 2. I do like this game, but man those vehicular sections were monotonous to me. It's cool but I don't grok the "best game ever!!!" sentiment it inspires.

Underrated:

Brutal Legend. It's an RTS/Action hybrid in the vein of Herzog Zwei (speaking of underrated...), Sacrifice, Giants: Citizen Kabuto, but when it came out everyone flipped out because they didn't want to take a few minutes to learn how to play it as anything beyond a simple hack-and-slash game. It's not flawless - the open world missions are often dull and repetitive - but the core gameplay is satisfying to me and as a heavy metal music fan, the soundtrack is impeccable

Freedom Force/Freedom Force vs. the Third Reich. To me these are the best models of how to make superhero video games - party-based, tactical combat with a lot of environmental effects and gorgeous Jack Kirby-inspired presentation - but when they initially came out people couldn't stop griping about the lack of online multiplayer and then City of Heroes was released and stole all the remaining superhero game hype. The Marvel Ultimate Alliance games kind of took inspiration from them but it's not quite the same. I'd love to see Irrational get back to this series eventually but it seems they're all about Bioshock these days.

Outlaws. There aren't nearly enough western games for my tastes and this is a good one but it fell into obscurity pretty quickly, just like every other western game prior to Red Dead Redemption.

Tron 2.0. That Tron Legacy movie would have been better if they'd just adapted this game...