The only reason to limit yourself is an end of the world type event. So my choice would reflect what ammo I think I could find. As of today I would have a 9mm pistol, 22lr, .223, .308 and a 12ga. The .308 could just as easily be a 30-06 and the .223 could be swapped for a .762x39 . If limited to one, it would be the 22lr. Only because you could carry tens of thousands of rounds, if you had to.