Alan0354 said:
I don't know enough about ballistics, is .40 better than 9mm for self defense? Is 45ACP better than both?
The simple, truthful, unbiased answer is as follows...
Generally speaking, 9mm Luger has the most penetration, but the least energy. .45 ACP has the most energy but the least penetration. .40 S&W is nestled snugly between 9mm Luger and .45 ACP, featuring more energy than 9mm Luger, but slightly less than .45 ACP, and more penetration than .45 ACP, but slightly less than 9mm Luger.
Which is "best" is entirely subjective.
There's no such thing as the ideal, one-size-fits-all cartridge for self-defense because people are individuals and thusly what some find suits them perfectly may not work out at all for others.
Case in point, some folks find the recoil impulse of .40 S&W to be detrimental towards speed and accuracy, but I find that I shoot it just as well as 9mm Luger defensive loads, with no noticeable difference in speed nor accuracy.
9mm Luger is currently the most popular duty cartridge worldwide, which it will most likely remain until the invention of the Hand Phaser, and it's no surprise why. Folks of all shapes and sizes shoot it very well because it doesn't have much if any felt recoil in full-size duty weapons, the grip size necessary to accommodate the 9x19mm cartridge fits most hands well, and the cartridge offers more than enough in terms of ballistics performance to stop a threat with proper shot placement, thus making it the ideal cartridge for outfitting an entire agency or army because pretty much everyone will be able to shoot it well, regardless of their size, shape, grip strength, etc.
It's an extremely well-rounded cartridge, hence why so many folks swear by it, with many even going so far as to push it as the illusive ideal, but whether or not it is so is up to individual discretion.
That's the most honest, truthful, unbiased answer that you are likely to receive, so be prepared for the distinct possibility of responses which essentially amount to; "Whatever I carry is the best because reasons and anyone who carries anything else is dumb!" Only they won't be anywhere near as quick about it, and will likely bombard you with links to a wide variety of things to support their argument.
Good luck.