Some of my favorite roads in the Black Hills - corners, corners, corners!!
In no particular order:
Spearfish Canyon - Roughly 45 miles full circle, there are a lot of corners (sweepers and some tighter ones) and the scenery is beautiful as the road twists and turns throughout the canyon, with canyon walls looming high above your head and a river guiding your way through the rock. There are several good stops along the road for hiking and sightseeing - one of which is Bridal Veil Falls. You cant miss it because there are pedestrian signs, rumble strips in the middle of the road, and a decent sized parking area across the road from the falls. There is a viewing platform for pictures and you don't have to actually 'hike' to see it.
Another worthy stop is Roughlock Falls. Turn towards the Spearfish Canyon Lodge on the gravel road. Go less than a mile and there is a parking area on your left. The path to the falls is moderately steep elevation, but not super long and well worth the views.
Spearfish Canyon - From Sturgis, the fastest way to get there is I-90 to 85 South to Spearfish, take 14A South.