I may be wrong, but I believe the term "best gun" originated from the English gun trade itself and English firearms "authorities". At one point in history, the "best" of everything was considered to be made in London(especially by the English upper class). Since the London gunmakers had a monopoly on the gun trade by royal decree, guns, clothing, and all sporting equipment coming from London was the "best" and everything else was considered second rate; Birmingham gunmakers being the bastard children of the trade. The idea of "best" however, was not unique to the London gun trade...Ansley Fox considered his guns to be the finest in the world; an opinion shared by Teddy Roosevelt who also owned English guns.
However, all of the criteria I've read that supposedly determines a "best gun" is simply a description of one made by a London gunmaker; a claim that would be challenged by those who own and shoot pre-war guns made by Austrian, Prussian, German, and Belgium gunmakers. Personally, I wouldn't give two cents for a gun I couldn't consistently hit birds or targets with despite the name on it. Have I adequately stirred the pot on this Joe?