Way too many to choose from, but I’ll say Apache Trail.
The Icefields Parkway between Banff and Jasper in Alberta is very beautiful.
Among American interstates, which are normally deliberately uninteresting, I certainly second the above nomination of I-70 in Utah.
Well it seems like a lot of people agreed with my choice of highway. Anyone interested in seeing some pictures I’ve taken while in the Sierras?
US 101 along the Oregon coast. I was impressed by the Astoria-Megler Bridge crossing the Columbia River.
Most of the rural roads in New England are very nice during foliage season.
US129 around the Tennessee-North Carolina border. It is called the Tail of the Dragon and it has 318 curves in 11 miles. Very popular among motorcycle and sports car people. The surrounding scenery of the Great Smoky Mountains isn’t too bad either 
I-70 through Glenwood Canyon, since QB has already posted about the Million Dollar Highway, aka Red Mountain Pass. None of the pictures I found at Wiki do it justice.
The Great Ocean Road in Victoria is pretty spectacular.
Utah 95 from Blanding to Hanksville - goes across southeastern Utah through some spectacular canyon country.
US Route 50, between Winchester, VA and Grafton, WV is one of my favorite drives (east of Winchester and west of Grafton, it’s just a basic four-lane highway). Before DC-area retirees started to move there and buy nice, new houses, it was even more wonderfully rustic and colorful. The cheap old motels and ghostly defunct roadside gas stations remind you that it was the main artery for traffic between north central West Virginia and DC (Grafton was a major railroad hub). The scenery is stunning (e.g., Saddle Mountain). But don’t plan on making time through it – the hairpin curves will make your car’s bumpers touch.
I have driven many of the roads mentioned here and agree with most of your selections. I would like to add one more road you should put on your list. California Highway 49. Runs through the gold rush country. I have only traveled parts of the southern section, but it is unbelievably pretty.
One of these days I want to take a vacation and see it all.
I’ll add rt. 132 around the Gaspé Peninsula in Québec. Awesome.
We took a self-conducted wine country tour in Portugal when we were there. We drove up the Douro River Valley from Oporto, and it was spectacular. The grape leaves had all turned color and the vines were loaded with huge purple fruit. It’s a very picturesque and rustic area.