Sales don't mean that much. If you go by sales, I bet porn movies dominate the "best" movies of all time list.
I think Crysis should be on that list, actually. As far as I know, it still has the best graphics of any game ever made. In fact, it will probably keep that title for a long time thanks to the current "make it work on any 5-year-old laptop" trend in PC game development and the lack of new consoles being in development. It's also the first Windows game to have a 64-bit version. Oh, and it was plenty fun, too.
I don't know about making a top 25 list myself. I've probably played a hundred PC games but there have been LOTS of games that I've missed, too. I never played XCom, for instance, or Rome: Total War, or The Sims. I don't want to go knocking games out of the list just because I missed them.
It might be more interesting for us all to nominate a whole slew of games that we think are contenders then have a big poll where we rate each game 1-to-5-stars OR mark the game as not played. Then take the average scores of those who played the game.
P.S. Where the Hades is Thief!?