Welcome WBF. This question has been asked at least once a month in the time I have been here.
The reloaders will say .44mag, .357mag, .45 Colt.
Cops a duty rated autoloader in a useful caliber.
Crotchedy buzzards will say a 100 year old triple lock.
Youngsters will say Glock, pick a model.
Myself a 9mm CZ 75 with a .22 Kadet upper. Joe
If I could only have one handgun I suppose it would mean I wouldn't be doing much shooting so I'd likely be happy with a .45 Commander. This subject comes up here from time to time and it invariably deteriorates into folks listing 5 to 10 guns.
As to the reason, I don't live where there are big bitey things so don't need magnum power - .45s are fine. I like 1911s and shoot them well enough but they are too heavy to lug around so a Commander fills the bill. I wouldn't have a problem with a .38 Super, but would prefer a .45.
I remember when I only had one handgun, for quite a few years: K frame revolver. The days of one handgun are long past, but my Model 66 is still the best handgun I have, for the first 6 shots. Can hunt up to deer with .357, small game with .38, cast lead bullets and reload (with nearly any powder), self defense, possible to carry.
For me its a toss up between an m&p 40 FS or Compact. Full size would have the longer barrel, longer sight length, Compact would be easier to conceal, and is capable of taking high capacity mags, but has the shorter barrel. I have the shield 40 and love it, but I think I would want the higher capacity if I were to only have one gun.
This is a tough question, I'm torn between two guns. If it were limited to one gun, I'd go with my Glock 20 10mm. I can load it up or down. DT has a 125gr copper load at 1600fps that mirrors the best .357 load. Underwood makes a 200 gr XTP at 1350fps for hunting. My target load is 950fps and a ***** cat.
My 4" 45 Colt Mountain Gun just missed first place, extremely versatile as well.
M&P 40 with an additional 9mm barrel and a few 9mm mags. Then I'd have 2 guns
I won't include the 357 Sig barrel because although it would be 3 guns, I personally have no use for 357 Sig (unless I pick up so much range brass that it justifies it, but I'm getting there).
You said "pistol", so I'm thinking autoloader, which rules out my beloved Model 10-5.
Only one? A nice 3913 with several mags. Famous for being rock-solid dependable and easy to carry and shoot, in one of the most common calibers in the world.
Mine would have to be my Randall General Curtis LeMay .45 ACP. It's a smaller, stainless steel jewel with ivory grips that I just love to shoot and it is easy to carry in my Milt Sparks Summer holster.