Leaving Avebury and its associated monuments out (too close to home) my other choices are:

First Choice: By far and away has to be the Ring of Brodgar. Last summer I made the long and tiring journey to the north of Scotland then over to Orkney. Something inexplicable happened when I walked up to the circle - perhaps a combination of the midsummer light and the crystal clear air, the experience was everything I had hoped it would be and I definitely plan to go back.
The Ring of Brodgar

Second choice: The King’s Quoit, Manorbier in Pembrokeshire.
Visited a few months ago and just loved the walk up the cliff path to a fairly small portal tomb overlooking the sea towards Ireland. An exquisite spot, even in the rain.
King's Quoit, Manorbier

Third choice: Aghnacliff in County Longford, Ireland. Home of my maternal grandparents. Haven’t visited yet but it feels as though I must.
Aghnacliff