Highway 33 from Reedsburg to LaCrosse is nice, especially west of Wonewoc
Highway 60 from Sauk City to Prarie Du Chien is more of a "touring" road (not extremely curvy) but is pretty.
Highway 14 from Madison to LaCrosse is nice
Highway 27 from Prarie Du Chien to Sparta is fantastic
Pretty much any county road in the box of Sauk Prarie, Prarie du Chien, LaCrosse, and Mauston is good. The area is mostly hilly, rural, farm areas with Amish farms and natural beauty.
Highway 35 along the Mississippi from LaCrosse to Prarie du Chien isn't a challenging ride, but is very pretty.
Highway 171 from west of Richland Center west to Highway 35 is also a beautiful ride.
I grew up in Baraboo and typically make a trip there (about two hours), ride the backroads around Baraboo all day and daylight runs out before the good roads do.