I've had a pretty varied career, and have seen several sides of this argument...if you want a guaranteed 1 shot stop, use an RPG...everything else is a gamble, but requires shot placement.
The current trend is towards 9mm....you can expect that to change (again) when the current group of trainers, writers, and LEO leaders change...in a few years, either speed and weight (40 cal and 357 SIG) will be king, or slower and heavier (45acp)...
9mm with 124 gr bullets, you have good penetration, and good speed-and a higher capacity.
40 cal you can play with speed vs penetration a bit more, but at the expense of recoil and lower mag capacity...same goes for the 357 Sig...I'd give the edge to the 357, but with both, you are getting more very snappy recoil.
45acp a bit slower, a lot heavier bullet, and good penetration...heavy recoil, but a good bit slower and gentler recoil.
So it comes down to...more recoil and fewer rounds, with the potential for better penetration on bone...or car doors....OR less recoil and more rounds. Or do what the FBI and current trainers are doing and choose the easiest to shoot well with less training and practice...as in how much can you afford to shoot in practice.
For me, I carry either a 1911 in 45 acp, or a G19...depends on where I'm going and what is going to conceal better.