I've ranked roads based on four criteria that I have defined. They are:
Twistiness: The amount and "tightness" of the curves on the road. a "1" is completely straight; a "10" is an abundance of super-tight 15 mph essess, hairpins, switchbacks, etc. A "6" is what most normal people would consider a curvy road.
Road Quality: The condition of the road surface. A "10" is a nice, wide, completely smooth road surface, with perfect signage and paint. A "1" is barely above gravel, full of potholes and patches, etc. A "5" is reasonably acceptable pavement.
Traffic: A busy road usually means no fun on the twisties. The more [slow] traffic there is on a road, the worse the score. A "1" means endless hordes of slow moving cars, SUVs, and motorhomes. A "10" means you might be un-lucky if you see four cars in an hour.
Scenic Impact: Although not really a factor for twisty road driving, this is still an attribute of a road's niceness. Completely jaw-dropping scenery gets you a "10" on this scale.
I generate two score values from the above criteria:
Fun-to-drive is a measure of how enjoyable a road is to drive from a purely motorsports perspective. This measure ignores scenery and places a heavier weight on twistiness and traffic. (The specific formula I use is: (twistiness * 1.7 + surface quality * 0.5 + traffic * 0.8) / 3)
Overall Score is a measure of the road taking into account scenery, and more heavily weighting the smoothness of the road. (The specific formula I use is: (twistiness * 0.95 + surface quality * 1.15 + traffic * 0.95 + scenic * 0.95) / 4)