I think the National Knockout is the closest gauge you get but it is a Knockout competition and one bad game and you our gone.
It also doesn't consider the strength of the league in total as well.
I have played in both the Home Counties Prem and North East Prem and were both serious standards.
If you consider Berkshire have won the Minor Counties the past however many years and the majority of their players play in the HCPL and as said above South North consistently reach major Trophies later stages.
Home Counties Prem is a serious standard.
To be honest, any premier league is a very good standard of cricket, and realistically more or less everyone in those leagues is a semi professional player in that they are being paid to play.
No way on earth you can say Surrey is the strongest, or Hants, or HCPL or whatever.