ArmA2 or whatever the current may be, reason being it's modding community, the mp experience in a closed community, every game is a different story to tell and open ended and not set to strict rulesets.
I could never pick something so basic, locked down and repetitive as some have mentioned, despite liking lots of games as such immenseley they don't have much else.
It couldn't be singleplayer for me either, friends and others bring endless enjoyment.