I usually avoid these discussions along with discussions of politics and religion, but what the heck;

In my opinion, 9mm, .40 and .45 (and some of the other oddball and older cartridges) can all do what is required whether for defense, law enforcement or targets, assuming proper ammo choice. Leaving aside ammo prices, which one is "best" depends on the gun, the shooter's needs and abilities and personal preference. If you are limited to non-expanding ammo, that is a different matter.

If we are talking about defense/law enforcement use, then for the sake of discussion I would generally choose a 9mm in small, light guns and the .40 in "duty" sized guns EXCEPT that there are some very nice Performance Center single stack .45's that are not available in .40 and I like single stack guns.

I am still quite fond of the .45 but am beginning to think there is nothing it does that the .40 can't do as well with maybe a couple extra rounds in the magazine, and in a 4" or shorter barrel, perhaps do better. I don't find the .40 in a 180 gr. is any more "snappy" in recoil than a .45.

In the end though, it's like asking what flavor of ice cream is best Your answer and mine may be different but equally valid.

I think we all need at least one gun in each caliber.