I voted AR since parts and ammunition are plentiful, plus its lightweight and its cartridge is reasonably versatile.
However, there is no one gun that will do everything required in a SHTF scenario. I would definitely agree that a .22 rifle is an excellent gun to have on hand. Subsonic loads are very quiet and have plenty of power for small game at moderate ranges. In less you are forced to move in a SHTF scenario, you're better off staying put; so no reason not to have more than one rifle.