A really good selection of tight hairpins, fast sweepers and a nice view as the road rises up from sea level to around 1000m. As this is rural Spain, you may need to pay some attention to farm animals straying into the road (although I didn't see any on the way). This route is ideal if you've just got off the ferry from Santander.
This route will put you and your bike through the most exciting roads you will travel. You follow the River Doura to its source in the mountains.