I think that one would also have to determine their needs and level of
paranoia to properly answer this question. Are you simply looking for something to shoot a
burglar with? Do you expect to have to
fend off a crowd from your
candy store during a riot? Are you planning for
the chaos after society collapses? Do you intend to fight off an attempt by
police or other heavily-armed group such as rival
drug dealers to take you into custody and/or kill you?
The above-mentioned shotgun is suitable for close-range intimidation as well as for clearing a crowd, but while a double-barreled shotgun is satisfactory for the former, only a pump shotgun is useful for the latter, unless it is a small crowd.
If you are of a survivalist mentality, then you require at least two, and as many as four weapons at a minimum; the shotgun, possibly a bolt-action rifle for hunting, and one or two (one small backup revolver) pistols for personal defense. The backup should be a revolver as it is the most reliable, and cannot jam.
(Another advantage to a revolver is that almost anyone can operate it. There is no safety to worry about, and if you hold an automatic pistol wrong, the slide can break your thumb.)
For serious defense, I suggest you get a minimum of five weapons; shotgun, a semiautomatic rifle, two or three pistols (including the backup), and a semiautomatic single-shot .50 caliber rifle.
I add the .50-cal, as that round (the same one used in a heavy machine gun) will defeat any armored vehicle short of a tank, including so-called "bulletproof" cars. (I am not advocating trying to defeat an armored vehicle, I am only pointing out that if you are going to be serious about it, you should arm yourself with a weapon that is able to do the job.) This can also be used in a "road warrior" situation where your enemy taunts you from a distance thinking he is out of range. A .50-cal bullet can travel accurately for over 2,500 yards.
Another reason to have a backup weapon is that under some circumstances it is better to remove the choice of your fate from your opponent. Depending on who is trying to get their hands on you, eating a bullet may be the easiest way out of the situation if you end up cornered.