Western Montana, Idaho and Washington are all wonderful with a ton of places to stop. The problem is that there are so many nice places, you would need 3 months to see them all.
I really love the section from Western Montana through the mountains to Spokane Wa. After Spokane it is fairly flat till Ellensburg (150 miles?), but then you climb the mountains up and over the pass and down into hell, I mean Seattle.
I live in Yakima and have traveled east several times and I have been in the other states along the way, and the best parts are in the PNW.