.22 Long Rifle
I have a firm stance on this that says, you should only carry a gun chambered in .22 if you've got absolutely nothing else to carry. After all, the best gun for you is the one you've got access to. If it's a .22, so be it. It shouldn't be your first pick, however. Having said all of that, smaller cartridges tend to lend the shooter to be more accurate at the range. If you can hit your target, you can hit an attacker.
Guns chambered in .22 are also great for new shooters who are just learning. It helps them get acquainted into the sport of shooting, and helps to aid young shooters learn the basics. Overall, this should not be used as a self-defense round, unless it is done as a backup.