Yes, it isn't a fair question you can really answer without context with what's happening in society. A balanced society, you will want a firearm for food and other matters. For the apocalypse, its an entirely different scenario.
For example, if you asked me this question 20-30 years ago, my answer would be entirely different than what today's answer might be. 30 years ago, anarchy and apocalypse was not on the radar really.
And to go even further, rules and laws that make you a felon change monthly depending on where you live. In an apocalypse, you could use a shorter AR with a pistol brace that can shoot mid range and close, and then rely on sling shots, or BB guns (not a firearm) to handle food and varmit stuff to feed you. It's quiet and they can be quite powerful, so you could own many BB guns of varying capabilities and still meet the single firearm requirement posed in the question.
The question should be more focused. It's like an open ended poll.
But since that IS the question, I too would keep my first pistol (which I still have), a Colt 1991 in .45ACP. 
The proverbial battlefield pickup. The enemy, whoever that may be, are more likely to be carrying Glocks than other makes. 
