There's been some statements in this thread that I have to express a difference of opinion with.
1. Quote: "Back during WWI, it was found the .45acp round to be most effective. During WWII, it was again the gun the Germans feared. NATO pushed for the 9mm due to lesser chance of lethal injury and being a common cartridge. As usual, the US government caved and went to a 9mm."
What study showed that the .45ACP was most effective (as opposed to the 1911 being a good platform for trench warfare)? More effective than what? Webley .455? German 9mm? Did someone establish a protocol and collect information on how combatants reacted to being hit with a round from a pistol? There are anecdotes; however anecdotes do not demonstrate stopping power, they are generally used to selectively bolster a particular position. The various versions of the Sgt. York story are a good example.
I've read a lot of WWII history, and have never seen anything about German soldiers notably having a special fear of the .45. With several million men under arms, I'm sure someone might have said something somewhere, but: "the gun the Germans feared"? As opposed to an M-1 Garand or a Soviet PPSh?
"Lesser chance of lethal injury". Again, I've never seen anything to document that. The desire to standardize calibers is well-known but there's been a long-running urban myth that Georg Luger invented the 9mm Parabellum to "wound" rather than kill, and this sounds like another re-hash of that.
2. The question has been proposed that one should state which weapon one would grab (9mm, .40S&W, or .45ACP), to confront an immediate threat. I believe the proper response is as follows:
First: the one I was currently trained and capable on.
Second: if currently capable on all, then the one that represented the best quality platform on the table; i.e., a new Kimber would trump a beat-up early Beretta M-9, a Beretta M9A1 would be preferable to an old Colt or Remington-Rand warhorse from the 40's and so on.
Third: if all were good examples, then I would take the one with the best type of ammunition in it. If FMJ were the only choice, I would probably grab the .40, because I've only seen that caliber with flat-points, and that MIGHT give it a slight advantage over round-nose FMJ.
3. Finally, there is the ever-popular "which would you rather be shot with?" question, to which I can only answer: I would strongly prefer not to be shot with any of them, because they are all very lethal, and I prefer not to expire just yet.
The winner in a gunfight will be decided by the fact of who gets the first vital hit in, and by the psychological reaction of your opponent to being hit if it isn't an instantly disabling CNS injury. Whether it's a revolver or any of the various auto types, a gun that fits your hand well-enough (and we are an adaptable species, within limits), places decently powerful hits quickly and accurately enough, and is combined with the training to enable you to do that to the best of your ability will count for far more than the thousands of articles written on "stopping power".