Narrow gravel road hugging side of mountains between Kuelap and Cajamarca (Peru).
Sheer drops on one side, and stone cliffs on the other; very sharp hair-pin turns on descent from Barro Negro Pass to Las Balsas; switchbacks are less abrupt up next mountain range and down to Celendin.
If you dare take the risk and travel along this route, then make sure to be driven by someone who has experience of the road. Staying on track here takes oodles of concentration. There's sheer drops virtually along the entire route and enough hairpins to make a whirling dervish dizzy. On top of all that, the surface is crumbling gravel and it's mightily narrow - just about enough space for one vehicle in some places.
