Smith & Wesson K Frame -- 38 Spl. Needs to be in the running because it was the standard US police sidearm for decades. But I like the idea of the SAA too. Back in the day, when I was a kid and the Saturday Matinee was about 25 cents, the Colt SAA in 45 LC was the icon, the only pistol you ever saw a cowboy use. (And of course the rifle was a Winchester 1894).
The cap pistols I owned were all copies of the Colt SAA.
-- But if you watch the vintage detective movies, the revolver is a Smith, and under-cover it's a Smith Snubby.