That's really tough, but I'd probably go with WoW as well. If we're allowed to have mods, then Civ IV probably wins out, though that in turn introduces Morrowind and maybe Skyrim. I'd also give strong consideration to SimCity 4, Dwarf Fortress, and Darkest Hour/Crusader Kings 2/Victoria 2/Europa Universalis IV.

Or Nethack. or Tetris: The Grandmaster Edition.