MGS4. The gameplay and it's scenarios were surreal, with every one of them different from start to finish, something I don't know if any other video game has accomplished. The mission that starts off in the middle of a group execution and lets you go about things however you like was such a mindsmurf.

MGO was also ahead of its time. I couldn't believe you could do all of those things in a multiplayer game.

Runner up is Killzone 2. It was the last game to push the multiplayer shooter formula forward, to me.