This would be my recommended route if you had to go from Los Angeles to Denver.
On this route:
-Las Vegas
-Hoover Dam
-Flagstaff
-Grand Canyon
-Horseshoe Bend
-The Wave*
-Zion National Park/Bryce Canyon (I think the road to this from Zion may be closed in the winter though)
-Arches National Park
-Canyonlands National Park
-Mesa Verde National Park
-San Juan Skyway
-Ouray, CO
-Black Canyon of the Gunnison National Park
-Leadville, CO (at 12,000ft, the highest incorporated city in the U.S.)
Not that I did this route myself but I have been to those places and I enjoyed them immensely. Some Coloradans here might have to clarify if this route is even possible during the winter as I'm sure some of these roads might close for the winter.
I don't have any knowledge about Oregon and Washington, but if I were coming down from Vancouver I would want to check out the Olympic penninsula, then head down I-5 from Seattle to Portland (be sure to check out the gorgeous scenery around the Columbia River). But once in Portland head west to the coast then take the whole coastal route down to LA. The coast from central Oregon to central California is a gorgeous drive as you weave in and out of lush forests, pastoral valleys and dramatic ocean cliffs. And since it will be in the midst of the winter rains, the whole route should be lush and green.
*The wave is a stunning geographic formation in southern utah, but its tricky to find. You need to secure a permit to go there and a little geographic know-how since there aren't marked trails to get there.