Well, the creator of ‘'Hockey Prospecting'' Byron Bader has you covered, as he recently released a report on the success rates from the biggest junior leagues in Europe and North America.
Taking a look at the draft from 1990 to 2022, 49.9% of skaters came from the Canadian Hockey League (OHL, WHL, QMJHL), however, the best success rate in North America comes from the USNDP.
I would take this data with a grain of salt as with most players being from the CHL, that also means a lot of late draft picks came from those leagues as well. It would be interesting to see a weighted analysis considering which picks were used and efficiency in that manner.