I think the first one (the list i gave is for that) since it seems to have the most tools and documentation available out there. After that Quake 3 seems to be

I think the first one (the list i gave is for that) since it seems to have the most tools and documentation available out there. After that Quake 3 seems to be the most popular, but that is multiplayer only (not that some people didn't try to make singleplayer mods though :-P). Also i forgot to link to the Quake Wiki: https://quakewiki.org/ https://quakewiki.org/