Wow! I don't know if there really is such a thing. It's going to have to be a heck of a compromise. As far as handguns go, anymore. Self-defense is my number one priority. It must be concealable but then again, I can conceal a fairly large gun, maybe not that comfortably, but doable.
However, I would immediately rule out all of the tiny pocket guns, and mini nines, and while I like my J frame 38's and my 9mm Shield, I would still want more gun.
For me it comes down to probably a Glock 19X 9mm or Colt Lightweight Commander in 45acp, maybe even a Glock 22 in 40 cal. Those are my 3 main carry guns. But I do have a Beretta M9A4 9mm which is growing on me and is currently in the shop, getting a bunch of Wilson Combat upgrades, including trigger, hammer, spring kit, and single side decocker-only, installed, and I am getting that darned threaded barrel cut off and recrowned. It just might become a main carry when I get it back, after acquiring a good holster or two for it.
But then again if it has to be my only gun, I might just opt for my Wilson Combat 5-inch gun, 1911, 45acp, which has a long and proven track record of extreme reliability with me, but that's mainly because I have owned it for many years. Really, the main downside to it is it's fairly heavy, but not so heavy that I would rule it out, and it's still very concealable in a Milt Sparks IWB holster.
The only bad thing is all of those semi's throw their brass cases all over the place, and I do handload, and if I can only afford one gun, I might be poor enough that I would need to reload my empty cases, plus I like the flexibility of tailoring my loads, which temps me to choose a revolver, in .357 mag or 44 mag. This would bring me around to a Smith and Wesson K frame size gun, or Ruger Blackhawk, or Model 29 Smith with a 4-inch barrel. And that brings me around to considering my second-generation Colt .357 peacemaker with a 5.5" long barrel. It has an action job with a superb trigger and is very accurate and the sights are well regulated. It's not really that heavy and could still be concealed and capable of handling most any reload I would want to stuff in it, from 38 special to reasonably heavy .357 Keith semi-wad cutters, or hollow points.
I can't choose, but I could get by with most any of the above, I might not be really happy, but I would still be well armed.