There are some people who will get hives from even considering the question, so they are excused from the following exercise. Select one handgun and one long gun for self defense, with the idea that these will be what you rely on permanently, come what may, till death do you part.
It will be interesting to find out what other people say. For me the choices are a .38 snub and a 12 gauge riot gun. The snubnose revolver is small, easy to conceal and vastly reliable in any good brand. Because you can carry it discreetly there is little reason not to have it with you, provided you have honored the legalities.
The ballistically correct and tactically grim crowd will roll their eyes and say, "Ken, that's not much of a gun! You need at least a..." I have thought about this and experimented over the course of many years. I conclude that people who say you can conceal a full sized fighting pistol in everyday life don't get out much. In some settings, in some modes of attire, it's just not practical.
Fortunately, we can find records of many incidents where the little .38 proved to be enough gun, for it was for many years the usual concealment piece for cops and is still popular in the un-badged armed citizen sector. It is not what you would choose to hold off zombie vampire hordes at Fort Mudge, but in the real world it will often work to break up an attack and allow you to extricate yourself from the situation. So long as you are alive for your arraignment, your self defense gun has done its job.
The .38 snub has some bad characteristics. It does not hold much ammo and it is difficult to shoot straight. The difficulty of shooting it owes to its small grip, long heavy trigger and the sights' short radius. Range practice, with close attention to shooting fundamentals, gets you past the difficulties, with the bonus that once you get the snub shooting where you want it, all other pistols are easier to hit with than they were before.
The riot gun needs less argument to defend it. It has a higher hit probability than any other personal weapon, twice that of a select fire rifle and nearly half again that of a submachine gun. It has a long history of stopping fights suddenly and its presence in your hands may dishearten opponents even if you don't shoot anyone.
The pump shotgun's only notable failure mode is the short cycle jam. Since it is an operator induced stoppage the operator can prevent it. SLAM the action open, all the way open. SLAM it closed. You will not break it, but if you operate the gun gently the shell feeding process may foul up, leaving you displeased--at the least.
There is one other operator problem, rare, but not unheard of. You must avoid loading a shell into the magazine backwards. Confirm, by touch or sight, that the shell has its base to the rear before you push it into the magazine tube.
Both of my guns are short range weapons. That does not worry me because nearly all justifiable self defense shootings are short range affairs, so I'm armed for anything likely to happen.
I would not argue at all with someone who prefers one or other of the small semi-auto pocket pistols in .380 or 9mm Luger, for these guns fill the same niche as the .38 snub. There are reliability issues with some of these pistols but I am told that the manufacturers are working those out. If you have such a gun and it's reliable, well then, good for you. It likely holds more shots than my .38, is quicker to reload and its trigger is better.
Notice, though, that these pocket autos are designed to fill the same niche as the .38 snub and some advertisements explicitly compare these guns to the J-frame revolver. The purpose of all these small sized, medium-bore guns is to keep out of sight, but to strike a reasonably powerful blow if need be, a role the .38 snub has filled successfully for many years. We have identified the job that needs doing; the debate is only about how best to do it.
Nor would I be critical if someone prefers a semi-auto version of the riot gun. For many years, few of the self-loading shotguns available were really reliable, which led to people steering clear of self-loaders for defensive use. A jam now and then does not matter if you are shooting at birds, but a gun that hangs up even once in a thousand times gives one an uneasy feeling if its job is defending you. Nowadays, though, there are several fine choices if what you want is a semi-auto fighting shotgun. The great advantage is that such a gun removes a step from the firing cycle, since you don't work the action manually. Your task framework is thus simplified to lining up your shots, firing, and stuffing more shells into the gun.
20 gauge shotguns, pump and semi-auto, are popular self defense choices with shooters who are of small build, or recoil sensitive for one reason or other. Again, no argument from me, though I think many of these people would be about as well served by a 12 gauge gun with reduced recoil buckshot loads.
What are your own picks?
To reiterate, the question is what would you choose if limited to one handgun and one long gun for self defense. I'm sure not everyone sees it my way, and some people, doubtless, have different circumstances that shape their choices. I totally get it. I would choose a larger handgun if I did not have to think about careful concealment, and if I lived on a big spread in Montana with open country stretching away for miles, the shotgun, with its built-in range limitations, would not seem like such a great choice.
I intend this as an open-ended question, symposium style, with no wrong answers, but please give your reasons.