Cader is probably the least used but most difficult. Big mountain, pushing bits, rocks, gates etc.
I wouldn’t say that Cader is particularly difficult – it’s just a big climb. The top is full-on scree and rocks, the middle is grassy singletrack and the bottom is gravel access road. (Ignore the summit which is unridable for most.) I rode it on a hardtail, most of the group I was with were on FS of various kinds. We all survived although the mountain claimed one cracked frame and one which was found to have cracked a swingarm later that week. I enjoyed it more than Snowdon but that’s possibly because it was far less busy with walkers.
We were quite a large bunch, about 12 in number iirc. We met a similar number of other riders on the mountain, who also post on here and put a video on. We also bumped into the warden who was laughing about the numbers of bikes on the mountain that day. He seemed to think it was a record.