Which gun do you need in a survival situation? The answer is "the right gun."
I think you need a variety of guns, because one has to plan for a variety of situations.
If the problem is other people, you need guns designed to stop other people.
If the problem is food, you need guns that allow you to hunt.
If the problem is an oppressive military or police force, you need guns that give you standoff range or superior firepower.
For Category A, you need pistols and/or a shotgun.
For Category B, you need a) a deer rifle, b) a shotgun for birds, and c) a varmint/small game rifle.
For Category C, nothing will suffice to give you superior firepower, as machine guns and fully automatic weapons are impossible to buy legally, and even if they were obtainable, the force opposing you would simply "call higher" -- make a telephone or radio call to higher headquarters, and bring in mortars, artillery, armor, or tactical air, which you do not have at your disposal. The best you could do against such a force would be to ambush isolated units or to attrit them with sniper fire.
My thoughts on the survival front started back around 2004. At that time, my scenarios -- the dark thoughts in the back of the mind that plague my sleeping, and sometimes my noonday thoughts -- involved other people. There would be a rise in crime, a threat against me from more than one person or someone significantly younger or stronger. My thoughts turned to "equalizers"; i.e., to pistols and revolvers. On that front, I think you need a .45 caliber pistol as your main line of defense. I also think you need a smaller pistol for concealed carry, as a Colt 1911 is a little big to fit in a waistband, or pocket, or in a shoulder holster. I own a .45 caliber pistol, though as I am left handed, I didn't go with a 1911 as the safeties seem awkward. I went with more of
a police model. I'm not a believer in composites, so it is steel. Heavy, too. I also have 2 9mm models. The first is
the old standby. Mine is extremely accurate, and only misfires if I use an aftermarket magazine. The other is
a concealed piece, which to my knowledge is the smallest 9mm concealable handgun that is all steel. It is reasonably accurate and conceals easily in a jacket pocket or in an inside-the-pants holster.
I am currently working on Category B weapons. For shotguns, I went with the
multipurpose Remington 870, in 12 gauge. I considered some bottom-eject Browning and Ithaca models (left-handed), but I liked the versatility of the Rem 870 and its multiple barrels. Deer hunting in Massachusetts is shotgun only, so I went with a model that gives me a deer barrel with iron sights. For non-Massachusetts deer hunting (and for elk, moose, bear, etc.), I chose another Massachusetts "Gun Valley" company and went with a
Savage Model 14, chambered in .308 Winchester. It does not come with iron sights in its left hand variant, so I am having them put on by the factory.
I do not consider Category C weapons practical or likely at the moment. My biggest current "scenario" is an economic collapse in which the police and military largely melt away. My concern is a) other civilians, and b) finding food. The Savage is, to my mind, a good hunting piece, and can be fitted with a scope for longer range hunting work.
If there's anything I need, it is some additional versatility. I'd add a revolver chambered in .357/.38 special, a 20 gauge shotgun, and a varmint rifle chambered in .22LR or .223 to round out the collection.