Morrowind. Especially with mods, it's just as beautiful as later ES games, and honestly much better story-wise. Between mods and OpenMW, I can reasonably look forward to playing it for years.
Lords of the Realm 2. The enemy AIs are entertainingly bad. Succeeding is hard without being impossible. I like how the audio is handled, and the narrator is funny. The graphics are certainly dated, but they were designed well and the game still looks good. It's easy to start and stop, I don't get locked into playing.
Cyberstorm. Doesn't work on my current PC, which is very, very irritating. I loved how the story unfolded as you went through the game. It was full of little touches that made the whole experience fun. The controls were good too, and the UI thoughtfully designed.
For console games, Link to the Past and Ocarina of Time. Those games are masterpieces.
Post edited February 10, 2014 by HGiles