That's quite a though one, Fox

Not sure really...first thing that comes to one's mind is of course favorite games, in my case FF VII & XII, Skyrim, Dark Souls, etc; but being stuck with being able to play just a single game is definitely bound to take it's toll no matter how much you like it. Out of those I think Skyrim (specially with the three DLCs) would probably be the better choice as it is massive and offers a huge amount of possibilities for you to role play virtually any character you want and I've put hundreds upon hundreds of hours into it and I still feel like playing it. But a factor in that, I think, is that I am able to take a break from it and play something else and then I come back to it. If I were stuck with only being able to play Skyrim, I don't know if it would work out that well.

Maybe a more casual type of game would be a better choice? The aforementioned Tetris is an interesting option.