4) GTA IV
3) Gears of War 2
1) Fallout 3
Basically, the first two are interchangeable because you could argue that Fallout 3 is the richest single-player game, while Left4Dead redefines co-op FPS dynamics. I love both games and play them as often as humanly possible.
Gears of War 2 is also on there for the multiplayer aspect (though I did enjoy the single-player campaign). The Horde, if you've played it with a full compliment of companions, is electrifying and hectic, much like all of Left4Dead (which is why left for dead is a better game).
Braid is a fucking weird but awesome game in old-school 2D platform style but with bizzaro Prince of Persia like time controls. The barren simplistic story is oddly fitting and I think that this is kinda like this year's Portal, except that Portal is probably better in several respects than the other games I've mentioned (I'm looking at you Fallout for your crappy FPS combat and you, Left4Dead because you're not nearly as amusing as GLaDOS from Portal).
We all know why GTA is good. I'll let you know when I start the missions and stop playing my own "Hit as many pedestrians as you can before the police can catch you" mini-game that's my go-to gaming style on all GTA games. Oh, except for when I use cheat codes to spawn boats on top of people/cars. That's a whole different awesome game-within-a-game.