Actually US 20 from West Yellowstone, MT to Ashton, ID in the winter is among my least favorite drives.
The stretch of 191 from Big Sky to the Gallatin Gateway is the worse around here. Between the sharp curves, the trucks, the animals that come on the road (especially moose), the ice, and river below...it's always an adventure in the winter. We do our best to stay off of it at night. There's a reason for all of those white crosses.