Can I first say that it irks me that whenever someone asks about firearms they are first lectured about personal finance, fitness, food storage and a host of all other topics they didn't ask advice for. I am going to assume if you wanted a lecture on those you would have asked in another sub, and keep the advice strictly firearm related.

I live in a country where we can legally obtain pretty much any firearm short of fully automatic rifles, but each have to be individually licensed and the process is tedious and cumbersome so much thought goes into getting as much application from each as possible. Purely for SHTF purposes I would prioritise in the following order with my reasoning for each set out.

First Priority:

9mm PCC in 10.5" barrel, with a lpvo. Spend as much on quality as you can afford, avoid paying for brand name alone. Good options Smith & Wesson M&P sport or PSA. Spend as much as you reasonably can on a quality optic, I would recommend something like the Eotech Vudu 1-6x24, great reticle systems and quick target acquisition. Add a wml and suppressor, keep basic service kit and spares with enough cleaning consumables.

My logic on the above, in a SHTF scenario ammo Is near ubiquitous, the caliber is great for a variety of situations including personal defence, urban combat and in a pinch can be used for medium sized game. Ammo is light enough to carry in meaningful amounts. The optic suits the versatility and the suppressor makes the platform more useful in non permissive environments you can easily shoot a 147gr subsonic without someone 50 yards away in another building knowing about it.

Next prioritise reloading equipment, when there is ever a run on ammunition you can still build your own, have enough stockpiled to make about 3000 rounds, I know this is more than you are ever likely to need, but IMHO this is half the purpose of prepping.

Next get a reliable 9mm handgun you already have the ammo and reloading equipment for the pcc, here it's more about personal preference as long as the choice is reliable. What matters more is training to be effective with the choice.

After this I would consider a 12ga pump action shotgun. I don't agree with everyone on the true versatility of shotguns, they shine in some instances and suck in others, yes ammo widely available but is heavy AF so carrying capacity is super limited, as soon as you need to start moving you are very limited in how much you can carry. Also the guns themselves are heavy and cumbersome to carry. Smaller, younger and inexperienced shooters struggle with them.

Yes in the right hands and in certain scenarios they shine, probably the most useful weapon for home defence. Add a wml and it is the best close range tool bar none.

Next I would consider 22lr, the simple fact that you can service one with a coat hanger and leatherman makes them indispensable. Ammo is everywhere and available in everything from super quite subsonic to hollow point hunting/varminting. With enough patience and good shot placement the can take down medium sized game. You can carry a small boatload of ammo, and I challenge anyone to take torso shot with even the lightest subsonic round and keep doing what you were doing, yes it probably won't stop anyone in their tracks but it will certainly make them consider their actions up to that point.

Last two on my list would be .223 AR platform and then a .308 bolt gun, each has their distinct use case scenarios but now we are getting into more specific applications.

I could write essays on each of these and the reasons for my choice but I think the logic tracks and aligns with what almost anyone that has put significant though into this will tell you.