I'm going against the grain here:
ArmA II. More specifically, Operation Arrowhead. If you're looking for a super realistic, truly open world military sim, it's simply unmatched. It's got some pretty big AI flaws in unmodded form, but there are countless mods, maps, and user creations that make it into almost a new game. No other game gives you half as much freedom as it does. Period.
With a friend or two, or a good server of everyone playing seriously, it's one of the most engaging experiences out there. So much deeper and more rewarding than the typical "run n' gun" military shooter.