Hmmm ... All of the calibers come in very nicely made platforms, for the most part. Lots of good quality ammunition available, using the best of the modern bullet designs in all calibers, too.
My first thought was 9mm, because of the accuracy and controllability of the caliber, as well as the overall acceptable size of the available platforms chambered for it. I've been carrying issued & personally owned 9mm pistols for many years.
Then, I'd say .40 S&W, because it generally requires a bit more of the user when it comes to recoil management, accuracy and controllability, but it also comes in nicely proportioned pistols which fit most small & medium-sized hands. I've also been carrying an optionally issued full size .40 S&W pistol for a few years, which I occasionally carry in my plainclothes assignment.
Then, I'd probably say .45 ACP. A personal favorite back when I got my first blued lightweight Commander, and then traded it for a steel Combat Commander. I've been carrying different personally owned pistols chambered in this venerable caliber as an off duty weapon since I first entered L/E.
Funny thing is, though, when I count the pistols in my safe, I own more .45 ACP pistols than I do 9mm & .40 S&W pistols
combined.
That must mean something ...
Also, when I was asked to select one of the new service pistols we're about to start issuing, which are available in both .40 S&W and .45 ACP (we're transitioning from 9mm as a general issue caliber), I tossed a mental coin and selected the .45 ACP model.
That must mean something, too.