ARMA 2 is the only true open world war game on market at the moment. You can play everywhere on a huge 225 square kilometer terrain and create infinite amount of different scenarios in the mission editor. (You can have hundreds of units from different factions fighting simultaneously.)
With the latest 1.04-patch the game is stable and mostly bug free considering the overwhelming size and scope of the game.
ARMA 2 is best played with a group of friends on a realistic mission. It includes built-in VOIP and JIP (join-in-progress).
This game is not your average corridor shooter where you move from checkpoint to checkpoint killing bad guys and then repeat. In ARMA 2 the missions can take hours (if you choose so) and you can use real military tactics better than in any other title release so far.