Disclaimer: all those western U.S. places sound & look amazing! I haven't seen any of them as an adult (yet), and would really like to take an at-least 2 week RV trip out there sometime in the next few years.
I was totally wowed by Niagara Falls. Not just the falls themselves from the top, but specifically Cave Of the Winds downward excursion on the U.S. side. It's like Tom described with the canyons, there is so much more to see/feel/hear/experience when getting down IN the feature. With COOW you descend down through a couple caves and come out on a series of catwalks mounted just below a section of Angel Falls. You can get pretty darn close (if desired) to standing right below a stream of the falls bouncing off rocks. It was a drenching-but-amazing experience! My wife and I did that side trip when we drove up to NY to buy our first RV, so it was an exciting venture for many reasons.