Best? There is no such thing except in the mind of the buyer. The word seems to have been attached primarily by London makers but also by a number of Birmingham's products. It was a marketing term. In the end you'll find the word "Best" is very similar to words like "Conservative" or "Liberal". They mean whatever the hearer wants them to mean.

I suppose if the word were used honestly it would be a statement by the maker that it is the finest they were able to produce when cost was not a limiting factor. (Even this falls short of a definition of the elusive term).

John McCain is my war hero.