I did the entire Northern Tier once, the western portion to Glacier N.P. a second time, and the portion of the Trans Am from Missoula, MT to Fairplay, CO. I also did the portion between Missoula and a little ways east of Jackson, MT last year.
Personally, if I could cross the country tomorrow, I would probably start in Seattle, follow the Pacific Coast route to the Northern Tier just east of Anacortes, take that to Glacier N.P., climb the west side of Going to the Sun, ride bak to Whitefish, take the Great Parks route to Missoula and pick up the Trans Am there. While I would be covering some familiar ground, I love the Cascades and other mountains in WA. Also, I did much of the Trans Am through OR as part of Cycle Orgeon. Other than McKenzie Pass (which I have climbed from both directions), a lot of it didn't impress me. I am not much of a high desert fan. I also want to experience Kansas.