Nethack. It's been around for 20 years now, and it's still a great game. It will run on practically any computer hardware, it's free software, it's got everything: challenge(maybe more than is needed), Liches, Leprechauns, a D&D basis and the extremely important OotS reference.

More seriously this game is so underrated because people these days care more about flashy graphics than challenging and thoughtful game play, and it has no corporation to make press releases. I'd say that it is as good as Baldurs Gate 2, not as good graphics, not as varied locales, smaller party, but so much more you can do (genociding Leprechauns comes to mind).

Also, it's had 20 years to be underrated. That helps if we are looking over 'all time'.