For wounding power, I'd say the .45 ACP is best; the .40 S&W is a little less, and the 9mm is the weakest of the three.
However, some say the differences aren't big enough to care which is strongest and which is weakest. That's a
personal issue only EACH INDIVIDUAL can choose to care about, when they see in light of which they shoot best with; if having more OR less rounds per magazine matters to them or not; if the price of ammo between said calibers matters so much, or not... etc. etc.
It depends on quite a few factors here... not small enough in numbers to make it simple in asking random people on a forum "which is best for ME" really. It (caliber selection) is a very personal issue.
So, as others said: Choose that which you shoot best with in rapid fire, and moving around to random, moving targets (thus more closely simulating a fast-paced, random, moving gunfight in the real world... though no training can prepare us for such a thing 100%).
If the price and extra or less wounding potential, etc. don't matter to you, then again: shoot what you shoot best with!
Anyone that says the 9mm, .40 S&W and .45 ACP "have the same wounding capability," are simply not in the know, imo. They're similar, yes, but not the same. Will these differences matter? That's up to YOU!
Me personally: I go with
10mm, because I see it as the best of all semi-auto cartridges. It penetrates DEEP with enough power to break big bones if they get in its way, and expands AWESOMELY in diameter at the SAME TIME! Out of a Glock 29 (sub compact 10mm model), even the most powerful factory rounds (DoubleTap) are not even as bad as .357 SIG out of a Glock 32, when considering recoil.
Again, that's just my taste. It's all up to YOU to pick and choose which is best for YOU! How? Go to the range, buy a bunch of different makes of ammo in each caliber you plan on choosing from, and which guns you plan on using as the platform... and see for YOURSELF!
It's the only way to get down to the bottom of this issue, my friend... and that is a fact in my mind.