Depends on your interests. For bucket list purposes, or measured by number of visitors, most of a top 5 for the West would be attractions like Disneyland, Universal Studios, Sea World in San Diego, and cities of San Fracusco and Las Vegas.
But if you are collecting National Parks, most of the heavily visited parks are in the West, NPS has the statistics. I'm guessing Yosemite, Grand Canyon, Yellowstone, Glacier and the Mt Rushmore National Monument. At some time I've been to four out of five of those, and would put only two on a personal "top five" bucket list. My cities list would also be quite different from a most visited top five, because for me, most visited works against their attractiveness.
Personal favorites among the western parks, for me, are Arches, Canyonlands, Glacier, Big Bend, and the Missions Trail in Texas.
Take "The West" out the the question and my answer is totally different. For RVing it would be mostly places in the Midwest and South, what everybody flies over or speeds past to get to The West. My top 5 for visiting or seeing in the U.S. are places I would not take a RV, or probably even a car, because they are either big cities or car-free areas (like Isle Royale and Mackinac Island).