9mm
It's interesting that so many have justified their response of .357 etc with comments such as, "I can kill meat with it", or, "it's the best self-defense rnd", etc.
But I have issues with that logic. The poll asks "if you had to choose one handgun, what would it be?"
The question puts no restrictions on what other guns one can have.
For a handgun, I'll stick with my Kahr K9 as main handgun, because it's light weight, easy recoil, reliable second shot. I'll take my SW 642 as BUG. Either will do for those times that I'm away from my beloved long guns.
Add to that my Rem 870P for SD (far more adequate than a .357), my CZ 452 in .22LR for small game acquisition, & (someday soon) a bolt action rifle based on the .308 (I'm guessing either .308 or 7mm08) for larger game (e.g., deer).
Wise is s/he who reads the question carefully, grasshoppa.
Nem