Well, before I declare the (imo) most overrated game I can think of, I'll take a stroll through the current contenders.
Halo: The first game started great. (Almost) everyone loved it. Then as more came out (starting with 2) it sucked, relying more on multiplayer as the popularity rose.
CoD: Same as Halo, except starting with 3
GoW: Not a bad game, but it's overrated. Mostly because of the gore I'm guessing (possibly story, but who knows).
GTA: Honestly, it's a sandbox game that forgot to give you a shovel until you're almost done with your castle. It's too 'realistic'. (annoying relatives, you die fairly easily, etc). But it has it's moments. Mainly when you have a helicopter tipped sideways in the streets with the fan turning; sending both cars and unlucky pedestrians all the way to Neverland.
Borderlands: Fun on the first playthrough (I will admit repetitive and a bit long), but after you've done it once, you've done it all.
Portal: It's mostly rated this good because it wasn't a game in itself. As Yahtzee said, more of a fill in for the Orange Box.
That's enough for now. Personally, I feel the most overrated game is... Mario. Judge me as you may. I find it a great game, but not excellent. And the remakes/others (Mario Galaxy, Wario World) are just terrible. Well, other than Luigi's Mansion, but you get the gist of it.
EDIT: And the most underrated game (imo) goes to... Halo. Nah, just kidding. It's the Total War series.