All these are superb and I can't choose between them:
Blue Ridge Parkway, North Carolina and Virginia (Appalachian Mountains)
Glacier National Park, Montana (Rocky Mountains)
Grand Teton National Park, Wyoming (Rocky Mountains)
Icefields Parkway, Alberta (Canadian Rockies)
Monument Valley, Arizona and Utah
San Juan Mountains, Colorado
Yosemite National Park, California (Sierra Nevada Mountains)
Zion National Park, Utah