I recently played an amazing DOS game where you have your country and you can declare war or peace with other ones, and i really enjoyed it. Growing up one of my favorite DOS games was Gobliiins 3, such cool memories!
Going by most influential, it'd have to either be Doom or Ultima Underworld. Both of these inspired entire genre's that still live on to this day (FPS and First-Person RPG's like The Elder Scrolls). Personally? Ultima 7.