Just one? This is difficult as I have so many to chose from, but I'm going to say it's probably the first Halo game. I had so much fun playing that, and it was my first game I ever played on a console. That, and so many memories of fighting the Covenant, the Flood, and even some friends in local MP. A couple of honorable mentions would go to MechAssault and BioShock. I loved those series so much.