I've uninstall from time to time, and of course I've changed of computer a few times so that's an obligatory uninstall.
Anyway, when it comes to the games that I've had longer on my disk it should be Heroes 2 and 3 by far, followed by M&MVII. I also get back a lot to point and click adventure games like Discworld 2 and Discworld Noir (especially Noir), Grim Fandango or Monkey Island 3, but those I normally uninstall them after replaying them.
Gothic and Gothic 2 are other good examples as well.