US421 from Bristol, TN (on I-81 at the TN/VA line) southeast to Sugar Mill on US321.
I dare ya.
This road is not only technical, it's flat-ass dangerous. It was my introduction to mountain riding. I did it on a V-Strom 1000. I would not attempt it on the Valkyrie I've been riding since (on which I was confident doing The Tail with my wife up behind me) and MAY someday try it on my recently-acquired 2005 K1200LT, but probably not.
Outside shoulders are drops of hundreds of feet through trees, no guardrails; inside of curves are vertical rock. Right turns have clinker crumbs scattered in the lane from four-wheelers getting tires off the pavement onto the clinker-ballasted shoulders. Violent elevation changes and switch-backs you couldn't put a self-respecting picnic table across sideways. Two mountain ranges, two decompression valleys along the way.
Sugar Mill is just a spot-on-the-map intersection. The nearest town of substance is Boone, about five miles east on 321. About 10 miles past Boone is US221, another high-tech road north. Somewhere up there is a place called Copper Ridge where there is an exchange with the Blue Ridge Parkway.