I'm sure the list is out there somewhere but i'm flucked if i can find it!
So far i have -
Avebury 300m
Stanton Drew 113m
Long Meg 107m
Ring of Brodgar 103.6
The Waun Maun circle is dipping in at 110m but i'm discounting that as there's little evidence for it bar MPP and his findings. Though the Ness of Brodgar website seems to kinda support it's place in the list.