One of my most enjoyable rides was on US 36 in Kansas. Most all farm country with small rural communities every few miles. There was just something about it that made me feel more connected to this country than ever before. Also the three US highways in south Kansas. For that matter, almost any non-Interstate US highway, and many state and county roads, will lead you through the unvarnished real American landscape. But once in awhile even a US highway is nothing but semi-urban blah such as US 40 in Utah.
Google Street View and Google Earth are superb tools for planning. If you're thinking about a particular road take a virtual ride first with either of those programs and you'll soon figure out if it's a road you want to ride or not.
Any one of the non-Interstate routes across the Rockies in Montana, Wyoming, Idaho, Colorado, or New Mexico will make for a great ride. The geography of the landscape varies so much it's hard to comprehend it all.
Decide what sorts of things you want to see, Google them, and make your choices. You'll just have to accept the reality that 80 to 90% of what you'd like to see isn't going to fit into your trip, and you will be bored by some of what you have to travel through in order to see what you want to see. Do what you can and don't fret about the rest. The result will be one helluva good time!