Okay, I'll play, and I'll provide an explanation for each choice too.
For this scenario, a new FEDERAL law states you're allowed 6 handguns, but only one per cartridge. No duplicates. Conversion kits not allowed. Spare parts must be bought with the gun. No spare parts after that. Reloading not allowed. Serial number designated to specific cartridge. IE; a .357 better not be found with .38's in it.
Calibers to choose from:
The ones I'd have to get:
.22 - This is the really hard one because ideally I'd want both a revolver and a Semi. I'd go for a revolver, and would be torn between a pre MIM S&W 617 and a pre MIM S&W model 63. I'd have as many parts shipped to the dealer as I could afford to play within the rules. If I could game the system I'd have a gunsmith remanufacture it before I bought it, mark it ".22 Rimfire", and include both .22 LR and .22 Magnum cylinders.
.357 Magnum - My heart says a pre-MIM 4" bbl S&W 686. My head says go with a Dan Wesson 15-2 pistol pack with several bbl lengths as someone posted above, or with a 4" Ruger GP-100. Either way, I'd get a medium frame .357 Mag revolver so I could shoot standard loads and the already existing factory Cowboy loads that replicate .38 Spl target loads in a .357 Mag case. I suspect that in this scenario ammo manufacturers would start offering .357 Mag ammo with names like light / cowboy (low pressure .38 Spl equivalent), medium (standard . 38 Spl equivalent), medium plus (.38 Spl plus P equivalent), and full (standard full pressure .357 Mag). I also suspect that S&W, Ruger, and Taurus would quickly bring to market their own versions of the DW easy change bbl system. See also theoretical lightweight alloy versions below in .38 Special comments.
9x19 mm - Springfield Armory XD-9 (not XDm) / HS Produkt HS-9 for near Glock durability, and the close enough to Ruger SR9 grip that fits my hand so well. If I knew they'd be close to as durable as a Glock 19 or SA XD / HSP HS I'd get a SIG P250 or P320 in 9mm. The modularity of those two models would allow for sub compact, compact, and service grip frame and slide sizes without violating the one SN per caliber rule.
.45 ACP - See above pistols in 9mm, change model / caliber. Despite its lower energy, the momentum of the big 230 gr slugs are close enough to 180 gr 10mm auto in terminal ballistics. It's also very likely that we'd see .45 Super cases factory loaded to full pressure and labeled as .45 ACP +P+ in this scenario.
.45 Colt - Ruger Redhawk. See .357 Mag ammo level and theoretical easy change bbl comments above.
The tough ones:
.380 - My factory hard chrome slide P3AT is my favorite summer deep carry gun. Being a portly gent in the hot & humid deep south that combination of small size and corrosion resistance is really tough.
.38 Special - I'd probably get a S&W 442 over the P3AT for long term durability. If, in this theoretical scenario, S&W were to come out with a no lock 386 with easy change bbls (as noted in the .357 Mag comments), or Ruger with a GP100 version of the same I'd probably skip having a .38 Spl revolver.
Alternately, if the same revolvers were offered in .38 Spl but could handle .357 Mag pressure levels then .38 Spl +P+ loaded to .357 Mag levels became a common and SAAMI / CIP accepted factory loading. In that case, since I live in the lower 48 states, I might skip the .45 Colt and use such a .38 Spl as a legal duplicate of my .357 Mag.
The if under particular circumstances:
10mm - If it was offered in the same pistols as the .45 ACP models I listed above, and various pressure levels of ammo were widely available I might consider this as a replacement for .45 ACP.
Calibers I'd skip:
.32 - I don't have any desire for it with tiny pistols like the Ruger LCP and Kel-Tec P3AT in .380.
.40 - It gets squeezed between 9mm and .45 ACP for me. It's also replicated by current light 10mm factory loads.
.41 Magnum - It gets squeezed between .357 Mag and .44 Mag / .45 Colt for me.
.44 Special and .44 Magnum - They can both be replicated by various levels of current factory loaded .45 Colt ammo.
Is this the kind of critical thinking you were hoping to generate David E?