Probably ArmA II OA.
Most people will probably go by looks and shallow graphics, but that loses its luster quickly. I prefer games with scope and ambition.
When I discovered just how amazingly intricate and ambitious it was. Nothing else even comes close. Scope, scale, and realism-wise, as well as the sheer number of stuff you can do, it blows pretty much everything else out of the water.
As a game, it has some fundamental flaws that keep it being less than perfect, but I'm willing to forgive many of those based on the simple fact that there's nothing else out there that attempts (and succeeds with) half as much.