Route 139 Colorado
I stumbled onto this route by mistake. I knew I had to go east to get to Palisade but I really wasn't concerned which road east to take. I ended up on route 40 east in Utah which dumped me off in Colorado. From there I took Route 139 south to Grand Junction and what an amazing experience it turned out to be. The route twisted and turned through the mountains. I was only jealous of the folks on motorcycles through this route. It looked like a biker's dream.
Route 95 Idaho
There aren't many options for going north in Idaho. There are only two or three roads that will get you from the meat of the state up into the panhandle. If you had to choose one route only, route 95 is the way to go. Through national forests and farm land, this route will also take you through Riggins which is a cute little town offering white water rafting tours to folks on the Salmon River. There's lots of winter recreation as well. This is a great route to get to Lewiston and Moscow.
Route 14 Washington
Along the north side of the Columbia River at the Washington border lies route 14, which I think turned out to be my favorite of all roads from my series of tours over the last 2 to 3 years. I started at the Bridge of the Gods which is more Cascade mountain geography and ended at route 82 on the east side of the state which is more high desert geography. The amount of diversity in landscape, climate and activities stood out to me the most. In the Cascade geography area of the route 14, the road twists and turns through tunnels and over small bridges. You're surrounded by a mountainous atmosphere with lots of trees and wildlife. Once you hit the high desert portion of the route, you will drive past wineries and other farm lands, see trains carrying produce to be distributed around the region and dams in the river that powers it all. This route puts a lot in perspective as far as want it takes to make the food and produce industry work.
Route 225 Nevada
After being on the interstates too much through Nevada, Idaho, and Utah, I decided to take route 225 north from Elko to Boise. The route changes to 51 when you reach Idaho but the beauty remains the same. You'll see plains, mountains, rivers and lots of photogenic atmosphere. This is a relaxing drive to take anytime other than in winter. I hear it can be a difficult and dangerous road after a snowstorm.
Route 74 Arizona
Also known as Carefree Highway, this route certainly lives up to its name. This will take you from just north of Phoenix to Wickenburg, AZ. The route has beautiful Lake Pleasant on it, which is always nice for a swim in the hot desert summer. If you enjoy desert scenery and lots of cacti, this route is for you!