S&W .38 Military & Police with a five inch tapered barrel and walnut round top stocks. Introduced the .38 special cartridge, amazingly durable and with some changes is still in production 114 years later.
Registered Magnum/.357 Magnum/Model 27. Introduced the commercial high speed magnum cartridge. The Heavy Duty and Outdoorsman also get an honourable mention.
S&W Model 19. First 'portable' magnum.
Walther PP/PPK Tradition Double action when it was a new fangled thing. Still popular today.
S&W Model 60. First stainless handgun.
Mauser C96 Broomhandle. For when you sit across the bar from Greedo. Also the first commercially successful semi auto handgun.
.455 Webley, the weapon that was the peak of English design, at the peak of their empire.
Hi Power. First wondernine, still an issued handgun for many western nations.
DWM Parrabellum designed by Georg Luger. Deadly Elegance. Introduced the 9mm cartridge to the world.
1911A1: Exceptional ergonomics, completely customizable. 100 years old in a few months.
Glock 17: First commercially successful polymer pistol, truly the Military & Police of auto pistols, although S&W have placed their brand on a very similar late arrival.
CZ75: The 1911 of Eastern Europe. a near infinite variety of factory and cloned versions, carried by police, military, citizens and used in gun games. Endorsed by Colonel Cooper.
Beretta 92FS: Exceptional reliability, mediocre trigger, fantastic Italian beauty. Needs a lot of oil to run. Just like an Italian car, but with reliability.
Colt Single Action Army: First commercially successful big bore cartridge revolver. Still a beautiful gun to shoot to this day.
Ruger Single Six: Brought revolver shooting to the masses.
Ruger MKII: Brought pistol shooting to the masses.
Interestingly, with the exception of the Broomhandle, Luger and Webley, all of my picks are still in production. There is a .32 Webley copy still manufactured in India, and periodically there have been attempts to produce new Lugers.