People get stuck on this one more than anything else. Some people will swear up and down the 45 ACP for pistols will drop a man in a single shot, and there is no other reason to carry anything else. The same goes for rifles, some suggest the 5.56 can’t drop a man. Luckily, this isn’t true. I suggest finding a caliber you can personally become proficient with and invest in that caliber.
I suggest this because shot placement is more important that caliber size. The vital organs of the human body do not know the difference between calibers. A shot to the heart will be a shot to the heart regardless of if it’s 45 ACP, 9mm, 5.56, or even 50 BMG. Dead is dead. You need a capable enough caliber, and I always suggest the minimum being the 5.56 NATO. Calibers like the 22 long rifle can kill for sure, but a rim fire is not as reliable as a center fire cartridge.
I suggest choosing a common, affordable, and widely available caliber. This will aid in acquiring and affording an ammunition stockpile.
Common calibers include:
Handgun Calibers – 9mm, 45 ACP, 40 S&W, 38 Special, 357 Magnum, 380 ACP
RIfle Calibers – 308, 30-06 5.56, .223, 7.62 x 39, 7.62 x 54R
These calibers vary in price, but are common enough to be found in any gun store, or big box store.
Outside of the 7.62 x 39, and 7.62 x 54R I suggest keeping to American calibers. There are a lot of great guns out there that are well built, reliable, and even affordable, but they may operate off an old round that may be difficult or expensive to find. These rounds include the 7.62 Tokarev and 9mm Makarov, good rounds in their own right, but they can be expensive and uncommon.