The one you are riding when the pavement ends!
I was out with some friends for a ride and decided to continue past Barlow Pass on the Mountain Loop Highway and on to Darrington. The first couple of miles was full of potholes so I had to ride carefully there. After that it was smooth hard pack with only the occasional light gravel and I zipped along smartly. It is not the most scenic 14 miles you will ever ride but there are a few spots worthy of some pixels. Plenty of side roads to explore if you find yourself there with a real dual-sport bike. Camping spots too, if you are so inclined.