I've always enjoyed Eversley Park Road on the 125, which sends you plunging
into the bowels of the Earth before hauling you back out again - as I
descend, I feel that the road climbing the far side of the valley is
parallel with the front of the bus. I suppose it's an optical illusion,
because the 3D views on WIndows Live Local suggest it isn't that steep at
all.

Shooters Hill on the 89 is another good valley, but you get a much longer
and grander view.