I choose the 9m.m., the .380ACP and the .38 Special.
I like the 9m.m. more and more as I get older. It has the combination of mild recoil in compact or larger guns (forget micro 9's), effective ammo and moderate noise and flash. When I started carrying a semi-auto, my issue load was 9m.m. +P+ and it got your attention. At the time, +P+ or at least +P was the only way to get a really effective 9m.m. round. Now, the premium bullets can do it without all the noise and fuss.
Even my wife, who has problems with most handguns because of arthritis, can shoot my BERETTA 92Compact effectively and comfortably..
If you cannot get premium ammo, then you can still get an effective round using +P or +P+ ammo.
I carried a .40 S&W for over 20 years as my issued duty gun and while it works great now, especially with premium bullets, you needed a large gun like the SIG 226 or BERETTA 96 Brigadier to shoot it well. At least I did. I shot the GLOCK 22 in .40 S&W and did not like it.
I went with the .380ACP for concealed carry. My GLOCK 42, simply disappears. It is reliable and marginally meets the effectiveness requirement with premium bullets.
The .38 Special was my third choice because of its versatility. You can pocket carry a 5 shot snub, but only with premium ammo as far as I am concerned. My 6 shot, medium size S&W model 15 with a 4 inch barrel makes a good all around defense gun when loaded with FEDERAL HST ammo. It is controllable, effective, comfortable to carry concealed in either a shoulder holster or pancake holster. Most people that I have worked with or gone shooting with can handle the combination of recoil, noise and flash while still hitting the target.
As a bonus, you can load .38 Specials in a .357 magnum revolver.
If I could have 5 picks, number four would have been the .357 magnum because it can do more than the .38 Special and is still easier to shoot than most of the big bores in my experience.
The S&W K-frame was just not big enough to shoot .357 magnum in regularly, but my RUGER Security Sixer's are perfect. Good triggers, just the right size so that recoil is not a problem and not too heavy like the GP-100 or 686..
If I were ever to carry a revolver on duty again, the RUGER Security Six would be my first choice.
My second would be my old S&W 681 ( a four inch barreled, fixed sight version of the 686). It shoot well when I went to qualify and had no problem with using the 125 grain jhp load. After we went to the BERETTA, I had it MAGNA PORTED and it makes 110 grain .357 loads feel like 38's. A great shooting, but heavy to carry gun.
I also like the .357 because I can use it in the woods, but would have to decide between convenience of a 4 inch Security 6 or a 5 in GP-100 or 6 inch 586. Weight has its advantages when shooting hot ammo, but drags you down when just carrying it.
Number 5 would be the often maligned .40 S&W, a really useful caliber in a medium size gun. I sold most of mine off and then wanted a cheap shooter that I could carry off duty or use as a car gun and bought a STOEGER 8040, which was much easier to conceal than my BERETTA 96 Brigadier. Funny thing, I liked it so much, I bought another. I have about $550.00 into the pair and no problems at all.
Jim