The Pacific Coast is definitely good, and I particularly like part between Astoria and Santa Barbara.
Otherwise, I've been partial to trips on the "edges", e.g.
- Haines, AK to Skagway, AK loop
- The Canadian Maritimes including along Bay of Fundy (both sides), Nova Scotia, New Brunswick and the road across Newfoundland.
- The coast of Maine, particularly from Bangor onwards and getting of US 1.
- The Alaska Highway or parts of Dempster and Dalton Highways.
I haven't done it yet, but high on my list are the Trans-Labrador Highway, particularly now that the gravel continues to Red Bay and also the Gaspe Peninsula.
Away from the edges, I'm also partial to the wide open spaces of the western deserts (in appropriate season), e.g. a nice loop through southern Utah. There is something about watching those vistas expand and slowly change that I still find fascinating.