The reason why most Americans have trouble with the term "best gun" is that its use was very parochial within the British gun trade during the vintage years, and at the time needed no explanation as everyone in shooting circles was 'brought up' with the terminology of the day and knew what it meant.

I suggest reading Chris Austyn's book "Modern Sporting Guns", for some insight into the use of the term. Especially Chapter 1: 'Best English Guns'!

The term means 'best quality', and generally speaking the configuration would need to be sidelock, stocked to the fences (although early guns often weren't), ejector, full coverage of fine scroll, no front lump protruding through the floor-plate, fine stock figure, chopper-lump barrels (if fluid steel), etc, etc. Then it would need to be finished with the best possible workmanship to qualify. Almost all 'best guns' were built to the individual specifications of the customer.

Not all British firms made 'best guns', and some firms like W&C Scott made just about everyone else's best guns in the early years!

Although most 'best guns' were SLEs, some Greener 'Facile Princeps' guns (the 'Royal' or 'Special' grades) were made up in 'best quality', and the WR droplocks sometimes were, but both these firms also made high-grade sidelock ejectors that they would have preferred to call their 'best guns' in the catalogues of the day. Likewise, Dickson also made a few SLEs as well as their marvellous round-action guns.

If you own a Holland Royal or Modele de Luxe, or a Purdey SLE, or a Woodward Automatic, or a Churchill Premier et al, you are undoubtedly the owner of a 'best gun' by anyone's standards, but the only way to be certain is to obtain a copy of the maker's ledger, and look for those magic words: "Best Quality"!