My vote's for Stonehenge. When I went, it was WAY off tourist season (January) and it was really cool. Got a murky, misty overcast day. Appropriate Stonehenge weather. :)

Also had the rope treatment--couldn't touch or walk amid the stones, but it was still pretty cool. I have a book listing all the stone circles in the British Isles & France. Still hope to get back there and visit a few more someday.