Of course Silbury would come up under a topic such as this but it is not really a hill is it. It is a Neolithic mound with some very specific reasons as why its a bad idea to climb it. Glastonbury Tor is a 'sacred hill' and hundreds of people climb it every day.
PS:
I made a return visit to the enigmatic Hetty Pegler's Tump (Uley long barrow) today. Now beautifully restored and open - though sadly some idiot has scratched graffiti onto the entrance stone, also inside. Absolutely no reasonable excuse available for such actions.