Long ago I decided that my main defensive handgun would be a Glock-19 in 9mm Parabellum. I had previously owned a .45ACP pistol (a S&W 4506). More recently I have resisted the popular migration to .40S&W. Herein I will set down the reasons for doing this which seemed the most compelling to me. This is all IMHO, YMMV, RTFM, and so on.
It is probably true that .40S&W and .45ACP can be loaded "hotter", i.e. pushed to higher energy levels with +P loadings, than can 9mmP. However, I regard +P defensive ammo as a fundamentally poor idea. It tends to wear the guns out fast if you practice with it, and besides +P practice ammo tends to be expensive. So most people who carry +P practice with standard-pressure ammo, but this is even worse, because the gun will handle differently in combat than it does in practice. Therefore I am confining this comparison to standard-pressure loads in the various calibers.
I have also decided to compare 9mmP only against .40S&W and .45ACP, and to focus only on the compelling diffenences. Whole books could be written going over the pros and cons of all available guns and loads, only to conclude that most differences amount to very little in the end. Anyway, here, at last, is my opinion:
If one follows the extensive literature on handgun stopping power, it is possible to defend either of two propositions: (a) that the best 9mmP, .40S&W, and .45ACP loads are all roughly equivalent in per-round "stopping power", or (b) that the larger calibers have a modest advantage. It really isn't important whether (a) or (b) is correct, because everyone agrees that speed and accuracy are more important than small differences in per-round effectiveness. For myself, I found that I can shoot 9mmP faster and straighter than I can shoot the heavier calibers, from a gun which is light enough to carry routinely. This is because recoil is less with 9mmP than it is with heavier calibers (given equal energy, a light, fast bullet has less momentum than a heavy, slow one - Physics 101).
In my experience, most people who are not shooting thousands of rounds per year will find that with the same amount of practice they will shoot better with 9mmP than they will with the heavier calibers. Also, 9mmP practice ammo is cheaper than .40S&W or .45ACP, so most people will practice more, and hence shoot straighter still, than they would have with heavier calibers. Hence, remembering that speed and accuracy are more important than small differences in per-round effectiveness, I conclude that 9mmP is the best self-defense caliber for me, and IMHO for other most people as well.