I recommend games from the 80's and 90's.
-This was also the golden age of space flight games. Wing Commander, Elite, and Freespace 2.
-If you are into FPS games, Quake 3 is still online.
-Starcraft and X-com are your best bet for a good strategy game.
-For the RPG players, Planescape and Baldur's Gate 2.
Be advised that retro PC gaming is NOT for graphic whores.