I thought ArmA 2 was awesome. Its a highly realistic military sim. The expansion ArmA 2: Operation Arrowhead was awesome to. Both games can merge with each other. In the mission editior you can place content from both games if you got both. You can do pretty much everything in the editor, if you know the right scripts and got some creativity.
I once attached 2 howitzer artellery cannons on the wings of a A-10.
The campaign is also great. I only got to level 4, and I loved every mission. Its really fun, but hard at the same time. There is also a armory mod, where you can test all guns and vehicles in the game. Multiplayer is also pretty fun, if you got a good team.