What's your pick for the "test of time" award?

Go on, I wanna hear about your nominations! Every person has that one game which they play to this date, and I figure seeing the choices of other people might motivate me and hopefully others as well to try out new games, which would be pretty excellent, having made it into this category.

I was torn between Deus Ex GotY and Unreal Tournament 1999 GotY, but ultimately voted Unreal because of the sheer replay-ability. I own the original collector's edition on CD which bought in 2001, and since then I have dumped more than 6000 cumulative hours on this game, I kid you not.

View attached image.