The last one is easily the best expossure with subtle detail where it's needed - ie controlled darkness. The 2nd is too blocked up. The first two have bags of negative space, if you sell commercially this can be good for copy text but as an image for an image's sake 3 is the best.
Thanks Gareth; it's worth the trip over (when it's good it's very very good.....). A useful way of checking conditions is the webcam on the website of the Ogwen Valley Mountain Rescue Team.
Thanks for the detailed comment Steve. My intent was the atmosphere given by the sky, not a literal rendering; I knew from the histograms that the hills would be pretty black, and it doesn't bother me. Even if I was using a camera with better ability to pull up shadow detail, or a Tryfan-shaped ND grad!