According to a recent Slate piece, it's... Solitaire? The article says that the various versions of Windows Solitaire are the most-used programs among Windows, and charts in particular the American workplace's ongoing love affair with the time-waster.
The article raises a number of interesting points — how the temptation of playing Solitaire at the workplace, for example, might have trained an older generation of computer-illiterates to use the mouse well, to minimize windows and swap between applications quickly (like you do when the boss is coming).
Among other things, what does it mean to the game industry that Solitaire is the most-played PC game of all time? If we're talking metrics, then that means that almost everybody is a PC gamer, right? And how might the gaming landscape have changed if every Windows PC came bundled not with Solitaire and Minesweeper, but a platformer or a point-and-click adventure?