We've enjoyed so many things, and so much has to do with the chances of the time you were there and what you were doing. I would not try too hard to list "best" or anything like that. I will simply mention some things I enjoyed, some famous and some not. Every national park is enormous, and I would advise setting aside many days to visit just one of them--don't try to visit four parks in four days or anything like that.
1) Arches National Park, in Utah is full of astonishing rock formations. One of them, Delicate Arch, is the state symbol of Utah but there is so much else to see. Try a Google Search on "The Three Gossips" and "Park Avenue Arches National Park".
2) In a sense it is not such a big place and there is not so much to see, but Devils Tower National Monument was one of the high points of one of our trips. When my wife was a teenager, her family went on a road trip, and there wasn't very much time, and they could see Devils Tower from the highway and her father would not stop to visit it... so finally, decades later, she got to make the visit she wanted to make.
3) We stayed almost two weeks at Custer State Park in South Dakota. It is next to Wind Cave National Park, which wasn't that great, but on the way back, on the highway we got stuck for fifteen minutes by a herd of buffalo crossing the road... bulls, mothers, calves, crossing in front of us and behind us.
4) Yellowstone National Park is probably the oldest and most famous of all US National Parks, and we loved it.