Currently that would be Tribes: Ascend even though it's in beta, since I have the access, I am playing the hell out of it, the main movement mechanic is unique and enjoyable.
I was about to list off like 10 games but then I forgot it said only 1 game. If I was going to choose only one it would be Battlefield 2142. I have never spent more time in any other game. (other than WWF No Mercy for N64 lol)