If conditions came down to having only one firearm, concealability will not be an issue. Portability, quickness, yes, and if it could be concealed, fine, but not as the primary characteristic.
Power, accuracy, and the ability to reach out and keep distance are the key factors. Neither a shotgun nor a handgun can do that as effectively as a rifle.
A rifle can do close quarters duty also, and penetrate barriers.
A folding stock and a 18"-20" barrel, reliable, low maintenance, magazine fed semi-auto is the all-around single weapon. When our troops use something else, it is for special purposes, not all-around use. Make mine an AKM, 7.62x39.