I would say "Best" guns are considered to be London made guns.
Boss, Purdey, Woodward and Holland are considered all Best guns.
Occasional you will see some boxlocks described as Best quality and indeed they look fantastic. Purdey and Holland made lower quality grade guns as well but Boss and Woodward never did.
But it all comes back to the fantastic four I have mentioned.
One would say that it has to be stocked to the fences and without any barrel lumps protruding through the bottom of the receiver to qualify as Best.
But Boss made guns not stocked to the fences and they were considered a Best gun.

Mike Proctor