Stonehenge for me.
Source: https://en.wikipedia.org/wiki/Stonehenge
While I wish I had the chance or opportunity to visit other ancient, monolothic sites such as this Stonehenge is the only one I have ever been fortunate enough to visit.
As far as visiting it again goes, I'm not sure I would.
I visited Stonehenge over 30 years ago, before they put up in the barricades and rope fences to keep people back. Going there again now would be a pale imitation of what I experienced the first time.
If given the opportunity I would visit a different site before seeing Stonehenge again. If however anyone were to offer me a free trip there I would definitely take them up on the offer.
LOL