HG/SS for an RPG introduction. No crazy generation mechanics like dynamax, mega, or terrastilize that you need to learn. Yes full support for doubles and singles. G/S has a well designed difficulty curve with lots of extra content HG and SS expand on it.
Champions if you just care about the competitive battles and not the RPG elements. Straight to the point PvP only, no breeding and farming required. Format based (they call them Regulations) so there's a limited selection of pokemon+items in the game with an evolving and changing banlist.