This mainly applies to a "hit the fan" situation......but isn't a bad option anytime.....ammo is the key. Gun without ammo is just an expensive club to wave around, and not a very good one at that. Limit choices to a select few, then pile up the components. Stack it deep. There won't be any more ammo coming anytime soon. You are on your own.
1. Along lines of "beware the man who only owns one gun", first choice is a 12 gauge shotgun. More ammo options than all other shotgun options put together X10. With right reloading setup and components (from light load of dusty bird shot to heavy load of #00 buckshot) can take down anything on the wing from carpenter bees to black buzzards. From chipmunks to grizzly bears. Few things are going to strike terror in mind of a home intruder than sound of pump shotgun racking in a shell in the dark.
2. 22 rifle........for harvesting game up to deer sized animals......and no joke if you get plugged by one. Ask President Reagan......and victims of covert snipers all over.
3. Centerfire rifle........any caliber suitable to your area.........any caliber at all as long at it's a 308 Win bolt gun. With right reloading components, any game larger than a 22 rifle would be used for...........all the way up to grizzly bears. Accurate, and effectively lethal out to 1/4 mile or more. Covers the whole waterfront with bullet options...........and longest barrel life going... typically 5K to 10K rounds.......take care of the barrel and a 308 will love you long time. Being one of the most popular civilian calibers, plus 7.62 NATO, means ammo and reloading options galore. The exotic boutique stuff will go dark in blink of an eye.......and stay that way.
4. If you need a handgun, make it a 22 pistol. A portable weapon that uses same ammo as the 22 rifle. Double duty ammo. Better have 5 gallon buckets of it. Can't reload 22.
5. Last one will be unique to me, but is on my list........high powered, high quality........air rifle. Along lines of HW77 or Hw97 or Air Arms 200.........in .22 caliber. No joke and suitable for dispatch of animals all the way to coyote sized........could take down a deer.......or harvest any sized bird or small game animal........and requires no powder, primer or case. Just the pellets, which compared to any other weapon, are dirt cheap Pennies per pellet. Could probably stock 5,000 of them for under $100.
What most may take for granted....until you need it.......are cleaning supplies to take care of these weapons. Cleaning rods, solvents and patches for the barrels. They won't take care of themselves.