Depends on what you call "one gun" I could have a Glock with 9mm, 40 cal, 357 sig, and 22 slide and it would still be "Federally one gun.....
With 1911, they make single shot wild cat caliber slides..... even 308 if I remember the craze correctly.....
Then add a CCU and you've got a rifle.....
Still with only one handgun frame....
Though in the spirit of the question..... G17 all the way.....
I'd say it would be my Beretta 92FS or any full size gun with a metal frame. You never know when you might need to bash a zombie in the head. A polymer fram just wouldnt cut it.
Ruger Super Blackhawk .44 magnum...Duh??? Is there any question? So what if people can see that youre carrying it. Have you ever sassed anyone that you knew was carrying a .44 magnum? Thats a no sass policy....
As I have said before my S&W 625 is the most versatile handgun I own. It can handle 150 light loaded wadcutters up through the very hefty 460 Rowland, will serve for rabbits through deer. Carries easily as well in my Kramer Crossdraw holster.
Glock 19 for me. 4 Pieces when stripped to clean. Easy to change out parts. You can have mags with 15, 17 and 30 round capacity. It's also easy to conceal if I don't want someone to know I have it.
Also, this is the gun I've had to most trigger time with and what I feel most comfortable handling.
Plus the Glock just takes a beating and keeps coming back swinging...
451_Detonics makes a good case for the S&W 625. Versatility is the key. The only thing you sacrifice with the 625 is capacity, a pistol with higher capacity magazines would make a better combat weapon. But all things considered I have to also go with the S&W 625.