It would depend on my circumstances. With my current circumstances, probably a 9mm or even a .380. For other circumstances in which I've lived, it might be a .357 or a .45 ACP. But both my father and my father-in-law somehow managed with only a single .22 rimfire each and both lived in the country. So I don't really like to be dogmatic about such things.
At the moment, I actually own two 9mm automatics, so while I may not be dogmatic, I am at least decisive. Sometimes.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.