From reading McIntosh's "Best Guns", my understanding is that a Best gun is the best a gun can be made, without sparing expense. Built to quality, not to cost. WW Greener would certainly argue the superiority of the box lock design, and there are some Greener guns designated as "Best". The Westley Richards hand-detachable lock guns ("droplock") also get a nod as a best. Some people would throw the Dickson round action in there, too. I'd also take a Purdey hammergun over a Best sidelock by some makers. No, I don't think you can say "Best" is exclusive to sidelocks.