Going with the war simulators, I realise some of these have been mentioned but I'm going on with it anyway.
ArmA II:
This is the ultimate combat simulator, simulating anything to do with combat you can think of, with mods to cover anything you don't think of.
Bullet drop, wind, suppressive fire, realistic damage thresholds, the traditional broken up voices traditional to the series, ArmA has it all.
http://en.wikipedia.org/wiki/ARMA_2Silent Hunter:
The ArmA of sub simulations, this is an AMAZING game.
You can play a mode in which you can just spam torpedoes at something and guaranteed a hit, or you can take the harder route and have to manually aim at, identify and sink each ship.
You are given a randomly generated campaign and sail around the world (literally, the entire world) trying not to run out of fuel, being blown out of the water by planes or sunk by a mine.
This links to Silent Hunter IV, in my opinion the best in the series.
http://en.wikipedia.org/wiki/Silent_Hunter_4IL-2 Sturmovik 1946:
Many different IL-2 game in one, 1946 has so much content. There are loads of planes (although they are not all flyable), ships, objects, cars, tanks and other things that have no relevance to a flight sim.
Also with randomly generated campaigns across the world, you can fly bombing runs against Stalingrad or fight to retain Berlin.
Forgotten Battles forms the main content of this game:
http://en.wikipedia.org/wiki/IL-2_Sturmovik:_Forgotten_Battles