Tough limitations ... but I've also seen similar threads where a "single gun" question became a gun-for-every-occasion thread ...
But ... I'd have to say it's my 3913.
Sure, I REALLY like .45 ACP ... and like all of my handguns, both revolvers & pistols ... but if I was going to be limited to a single handgun, I'd dedicate it to close range personal defense use, and pick a 9mm ... and that would be the 3913.
I've always considered the 3913 to be pretty much state-of-the-art for compact 9mm pistols.
Decent DA/SA transition ...
The shortest SA reset in a defensive centerfire pistol, making follow-up shots very fast ...
More than sufficient "defensive" accuracy ...
Minimal perceived recoil, and faster in shot-to-shot recovery out at 10-25 yards than with my .45's, .40's, .357 & .44 Magnums ...
The currently available JHP ammunition has been shown to be capable of potential penetration & expansion in actual shooting situations of an "average" range from 10-17"/.45-.65 ... and the performance potential of much of the newer & better 9mm JHP ammunition is very, very good ...
Acceptable capacity at 8+1, which is the approx "middle ground" of compact 9mm pistol magazine capacity ...
The grip frame angle it uses has been shown to be ergonomically neutral for a wide range of people, requiring the least effort to "point" naturally ...
9mm FMJ training ammunition is available virtually anywhere in the US at very inexpensive prices ... and it's a very common L/E caliber ...
MY 3913 has always functioned perfectly, regardless of ammunition brand & type, and so have all of the other agency & personally owned 3913's of which I'm aware ...
It has a Lifetime Warranty through S&W ...
Last, but not least, it's such a convenient size and weight for legal concealed carry ...