Dr Dirt,
This isn't my forum, but your question piqued my interest so thought I'd throw my two cents into the ring.
While I'm a 1911 kinda guy, I've always maintained that, if I should be allowed only one handgun, period!, that it would have to be an S&W Model 66 4" .357 revolver. A person could substitute any number of other models here (19, 586, 686, etc.)and be okay -- my favorite is just the stainless K-Frame.
Rationale? Ammo is available, in many variations and two different calibers, in just about any location on the globe. Not being reliant upon its ammo for its motive power (as is an auto), it will shoot everything from primed cases, blanks and shotshells through heavily-loaded magnums -- and do so repetitively.
In the right hands, and given the proper motivation of the moment, there is precious little that man fears that couldn't be taken or stopped with this gun.
It is small and light enough that it can (and, more appropriately, WILL) be carried regularly on the person without interfering horribly with other necessary gear.
It is simple and reliable; its basis has been manufactured in quantity, in various forms, for over half a century, meaning that parts will be available for it for a long time to come.
There are other reasons, but these are the first that come to mind. And, as an opinion, it is worth exactly what you paid for it (grin).
Regards,
----Mac Scott