Reader Discussion: What Single-Player Game Have You Replayed The Most?

Some single-player games are just infinitely replayable, and often we don't know exactly why. What's yours?

I have a few that I don't mind playing over and over for some reason. The original Mega Man X is one I probably play about once a year, so that was in the lead. Limbo I have played on every platform it has released for (except PlayStation 4, though I do have it downloaded). Also, I find Portal 2 weirdly replayable, despite knowing all the puzzle solutions.

How about you? What single-player game do you find yourself frequently revisiting.