9mm. Range ammo is currently readily available and fairly inexpensive. Good defensive ammo is the same way. There are so many good options out there for 9mm handguns that it just makes sense for someone who is new to handguns to go with a 9mm.
Flat out favorite? .45 Colt hands down. Reasons why? Well, history, both e calibers and mine with it. It was what I started shouting CAS with, one of. Two calibers I started reloading for. Well, technically the first, with a Lee hand loader. First two sets if dies I ordered were for 45 Colt and 45 acp.
The caliber itself has such a wonderful history going back to 1873 and tied to one of the best handguns eve, the Colt SAA. For real use, you can load it in so many ways. Black powder loads for authenticity and a lot of fun, roundball sand a pinch of red dot or bullseye for super light loads, standard loads will push a 259gr bullet to 8-900 fps easily and in a proper handgun you can load some pretty warm loads, with bullets up into the 300 gr weights. And, out of a rifle like a Marlin or a 92 Winchester design you can get even more velocity. I cast for it and have quite a few .45 caliber molds that helps keep costs down and adds to the versatility.
After years of shooting and tinkering with almost all of them, 9mm hands down.
It does everything I need from a handgun. Plenty of stopping power, low recoil, accurate, small case allows for high capacity, can be found anywhere, and it's cheap.
Other than family heirlooms, I am slowly getting rid of everything else. Once I shoot up/trade off my bulk .45acp, those guns are on the chopping block!
After much thought and careful consideration, my firm answer is YES!
Based upon whatever is close at hand and I can reload for.
I used to LOVE heavy recoiling big boomers, but as I've aged, gotten arthritic and a bit more fragile, the versatility to load up and down (and cheapness!) makes a big difference.
The exception to the reload for is, the venerable .22, just because.
Practically speaking, 9mm is my favorite. High round count, low recoil, and pretty effective. My actual favorite would be 10mm. 15 +1 in most handguns with 357 magnum power