In reply to Offwidth:
The UKC logbook system has recorded 230,000 ascents at Stanage, as opposed to 28,000 for Almscliff, or 280,000 for the whole of Scotland. Even accounting for the fact that people will make more ascents per visit than at many crags, the fact that it is the most popular in the UK is pretty obvious.
In the world - well I've not been to anywhere else that has that many routes or that many visitors, but then there are lots of crags I haven't been to.
Note to UKC logbook stats guru - most popular foreign crags would be interesting - I'm guessing Sella or Toix