|(source: Gun Nuts Media)|
(Note: Before it comes up here, too, let me clarify one thing: For the purposes of discussion, we're talking about service calibers for pistols — 9mm, .357 SIG, .40 S&W, and .45 ACP. No rimfire or magnum loads, and no rifle or shotgun rounds. I'm aware that plenty of bad guys have been put down by .22LR, .25 ACP, and .32 ACP, and that the 12-ga shot-shell is king of one-shot-stops, but let's stay on topic.)
|What? No .357 SIG? No .38 Spl?|
(source: Buckeye Firearms Assoc.)
The bottom line: Assuming quality ammunition, a quality gun, and good shot placement, there's no appreciable difference between any of the "service calibers". So carry what you want. The "best" caliber is the one that you are comfortable with, that you practice and train with, and — most importantly — that you have with you when you need it. There are advantages and disadvantages to each, but the best "carry caliber" is the one you shoot well and will actually, y'know, carry.
Is that a non-answer? Yes. Does that make it an entirely personal choice? Yes.
Will people still insist that the 9mm is the best because of higher-round magazine capacity and lower felt recoil, or that the .45 ACP is best because it leaves a bigger hole (and because John Moses Browning!), or that the .40 S&W is the best because the FBI uses it? Absolutely. (The FBI, by the way, is switching to 9mm.**) Gun enthusiasts become invested — financially and emotionally — in their preferences, and will continue to present their opinions as if they were absolute truth. They're free to do that. Liberty is funny that way. But most will admit that when the feces connects with the rotary air impeller, any gun beats no gun.
* - The threat of being shot tends to discourage most bad guys, as well. That the vast majority of defensive gun uses end with no shots fired is old news.
** - So, what's going to happen now to DHS' infamous order of 1.6 billion rounds of .40 S&W hollow points from a while back?