L4D2 is good, improves upon the original, but in some ways is less impressive because it breaks little new ground. Well, except the melee stuff which is pretty fun.
Arma2 came out this summer, a great game after a few patches have put down the worst bugs.
FEAR2 came out in Feb I think, good game, but somehow fails to achieve what the original did despite looking better.
I didn't get MW2 because I don't play MP that's not coop vs bots (ie SP with other humans on my side, like L4D), so $60 for that short SP game isn't worth it. I still don't have CoD4 or CoD:WaW because of the same thing, I'm waiting for them to be cheap.
DCS: Black Shark was released on DVD this spring, but was technically released digitally the end of last year (although I think few people bought it then), so you could put that in the running.
SF2 was released around the same time, with SF2V/E/I this year, but they're also improvements upon SF2, which is in turn an improvement over WoI, which is in turn...
For driving fans, Race On is good, but Race 06/07/GTR Evo/STCC/Race On is pretty much like the SF2 series...each release is standalone yet will merge with the previous. Each comes with a new set of cars and tracks as well as many from previous releases if you only bought the one.
I'm trying to remember what else came out this year...
Anyway, I'm not sure what I'd consider the best!