I'd say the best In the northwest for sheer difficulty.
90% tight forestry work, with a small but difficult hill climb section. No deep water but some lovely bogs.
If you like rock crawling or peat bogging go to Cowm, if you like driveable forestry, deep gulleys and water, go to parkwood, and if you like spending a day thinking how the hell am I going to get out of this place, go to silverdale.
Bear in mind the site is not maintained, so it is damaging and I say again, difficult to the extent of frustrating sometimes. I spent about two hours recovering a half disappeared 90 and then there was nobody left so I had to winch my own way out!
In fact that's the case in point, you won't get far without a winch in the winter, and simexes are required (my tyres are **** which is my major failing at silverdale)
Cheers