I'll reiterate that just about anything from LucasArts or Apogee is a winning pick, they all get my vote.
Also could go for Lemmings, Civilization, Oddworld, Space Quest, Lost Vikings, You Don't Know Jack (not really 'old' I guess, but hard to find and not always stable on newer PCs)
And for one out of the ether, Microcosm. http://en.wikipedia.org/wiki/Microcosm_(video_game)
No doubt someone has mentioned something I haven't thought of, lots of these folks know what's good! :)