Best indicates self-opener side-lock ejector stocked to fences with under-bolts not penetrating bottom or action the so called trigger plate. It will have what they call 100% engraved action with either 'rose and scroll' a la Purdey or more pronounced acantus leaf motif of Holland. The most well known are Holland & Holland Royals and Beesley actioned Purdeys. There is a member on this board who has remarkable system for determining value of English guns based on name associated with specific maker. As you expect name alone counts for thousands of dollars in price difference.
There is well founded suggestion that if one does not know a lot about English gun trade they should pick Woodward side-lock ejector with full coverage engraving, hint.
Sorry, no, it's not that simple.
Some boxlocks by Greener and Westley Richards were certainly bests; more than a few unengraved Boss and Purdeys were still bests. Quite a few london bests were not stocked to the fences. What do you do with hammers? It goes on.
A definition of "best" is just not that simple.
