I once did the descent off Pen Y Fan heading south, along the ridge of Gwaun Taf and then before you reach the forest you head down a trail to the little reservoir on your left. I would certainly recommend that!
It was fantastic as the trail along the top was just enough gradient to keep your momentum up but with some pedalling here and there, so some quite good flow to it in my opinion with some good little technical bits, it is a kind of sandy moorland type trail and feels really cool as you ride along the edge of the ridge. Then when you head down to the reservoir it gets really technical!! You can even choose to ride down what looks like an old river bed if you want some extreme tech! Lasts for ages too since the gradient isn’t that steep until the end.
Highly recommend it… but ONLY in the dry, I can imagine it is super laggy andt here is a lot of resistance along the top trail if it is really wet = one big pedal.
Route map and some photos here
No specific pics of the trail but that last photo shows where the steep part is i was talking about.