Wow surprises me no one mentions a .22. I would choose a nice 22, any of them I have a Weatherby and Browning but have had some others. For 98% of what we have for game (if it were legal) a .22 handles it. Most anyone can stock up 10,000 round of ammo easily, not as easy with other calibres and if SHTF a whole bunch of .22s hitting you is going to at least make you think. If I only had one rifle to provide food and protection it would be the 22. I have lots of other calibers but a .22 is the ultimate rifle..
5.56(.223) is a .22 basically