I don't know if it is possible for me to pin point a single site as almost every one I have visited had something of great interest to me or brings back good memories.
I am with Littlestone on the Avebury complex. It has been a big attraction to me for years and I am very lucky to have it on my doorstep. However, I prefere to visit outside "tourist hours" and avoid Avebury itself all together in the busy season.
Outside this, I think Belas Knap is probably my fave' or possibly Chun Quoit, or maybe Stoney Littleton or......hell, I don't know and what's more there are still thousands for me to visit yet!!

:o)

Chris