The 10 miles of Harper's Ferry Road between Sharpsburg and the river is - when there's no traffic and no crud in the corners - one of the most entertaining roads I've ever ridden. It's not a wide-open-throttle sweeper-fest like the MDH or the CS, rather it's an insanely technical back-and-forth, up-and-down ride that demands proper corner entry, brake usage, and gear selection.
There are a ton of driveways on it, there's often buses loaded with people there for tubing on the river, and there's no passing zones for when you inevitably get caught behind slow traffic......But catch it in the early afternoon on a weekday when there's no one around and there's no sand and gravel in the corners......It's just freaking magical.
FWIW, there are a ton of fantastic roads around here, many of them longer or more scenic, many with far less traffic and other obstacles, but whenever I have out of state riders visiting, Harper's Ferry Road is the first place we go.