I chose .40, 9mm and .380:
9mm: reasonable diameter, high pressure, bolstered in the last 30 years by much improved expanding bullets. The .308 Winchester of handguns: jack of all trades, master of none. Probably the best caliber for those who do not reload as it's (normally) cheap and easy to find practice ammo and good SD ammo.
.380: Loaded hot, it'll get nearly 1/2" of expansion and 12" of penetration...the bare minimum to meet the FBI criteria. My favorite handload is a 115 grain HDY XTP at 1050 FPS from a 3.6" barrel. Why is "barely passing" ideal? It's ballistically sufficient, and can fit into blowback handguns. My P232SL with black G10 grips, Tritium night sights and an 18 lb Wolff spring is my favorite carry gun. The huge downside is that the .380 is rightly maligned for substandard performance when using bad loads from very short barrels. A 90 grain bullet at 800 FPS from a Bodyguard or LCP is a poor caliber because the bullets will not reliably expand, and even then lack the velocity to adequately penetrate. Probably best left to handloaders, though Underwood 90 grain XTP loads are VERY good (they'll reliably expand after denim to 0.4" and penetrate to just a shade under 12").
.40: Almost replicates the terminal ballistics of a .45 but with a narrower bullet and higher pressure. I use 200 grain HDY XTPs which get 900 FPS from the 3.9" barrel in my P239. I can't make up my mind if I prefer "plenty of stopping power" or "just enough" (see above), but having two carry guns isn't the worst thing in the world.
Odd thing is, .357 Sig is overwhelmingly my favorite pistol caliber, but in a carry size gun, its ballistic advantages turn into a deafening muzzle blast with 9mm +P ballistics. But with a long barrel, it can compete with a 10mm (147 grain XTP at 1360 FPS) or a 7.5 FK (90 grain copper at 1950 FPS).