Skyrim was a less vaired Oblivion which was a less varied Morrowind. COD4 was a completely new experience in the FPS format. The every game is WW2 was past its prime and this was a shock of modern exactly when it was needed. And with that brought the best fps combat ever. Gripping story (with a fuckin NUKE) and touching first person scenes, which nothing has come close to reimagining. Games kill you every other level now days, but back then, that helicopter scene, man.
And I haven't been as attached to multiplayer since Quake. In fact, it was as good as pre-quakeworld Quake. And no consequent release comes close in that regard. It's easily my GOTG, but its also Number 2 of all time. I actually ponied up for live because of it. Went through a couple of xboxes because of it. All totally worth it.