If we get access too all future updates, Dwarf Fortress is my clear #1 choice.
If we also assume that the game will forever have an active enough userbase that lack of players won’t be a detriment: Infinity Wars as my #2.
If we assume no updates, no mods and no guarantee of users: Tales of Maj’Eyal and probably some 4x or grand strategy game, it’s a tossup between Civ, Gal Civ, and Endless Legend, maybe even one of the Total Wars.