This is a hard choice.
I really want to say Modern Warfare because it changed FPS games for the rest of the generation.
But playing all these military shooters lately is making me go "meh".
I love Skyrim through. It has it's flaws but it has one of the biggest modding community. People still play it 2 years on and it sparks creativity among gamers.
I voted Skyrim. I'm sorry.