I think it's a hard question to narrow down. For me, it would be a "general purpose" handgun. My choice would be a .357 - gives the option of more than one ammo. Probably a 3 inch barrel - easy to CCW but still accurate enough to plink, shoot critters on the farm, HD, etc. Sort of an all purpose gun. Of the top of my head - probably a 3" 357 Ruger SP101 or a 3" Smith & Wesson Model 60.
I really don't think that any one handgun fits all of a person's needs though. Otherwise, there would only be "one in the herd" . . . I'd wager that most here have more than one in their "herd"?