I certainly endorse several of the choices so far but I don't think anything is likely now to erase this one as my personal significant spot:-
Whorlton Bridge over the glorious Tees.
It's a fabulous and very old bridge, close to my ancestral home; I was two-thirds of the way round my first 400 (a solo DIY) and appear still to have been able to use the big ring. The bike (on which I started audaxing on one of Don Black's rides) has since been stolen but the memory never will be, except by the curse that steals all memories, in which case, I hope it will be one of the last!
Peter