A few months ago I would have said MGS3 but now I see it as the best game of it's time. I enjoyed the Remake but the gameplay doesn't hold up, too stiff.
Currently I'd say my fav game is Witcher 3 but it's been a while so something similar might happen there.
Also Gears of War on 360. Playing through Xbox Live with a headset, talking about life, the universe and everything while chainsawing people was a game changer for me.