I've been to 49 US National Parks. My Top 10 are:
1. Yellowstone - astounding thermal features
1. (tie) Yosemite - absolutely stunning
1. (tie) Redwood - I find the massive old growth redwood forests to be one of the most gorgeous sights on the planet
4. Glacier
5. Bryce Canyon
6. Canyonlands - one of the most underrated national parks IMO
7. Grand Canyon
8. Olympic
9. Zion
10. Big Bend
Honorable mention: Crater Lake, Arches, Badlands, Mt. Rainier, Sequoia, Grand Tetons, Denali, Kenai Fjords, Acadia
Worth visiting but underwhelmed by:
Pinnacles
Cuyahoga Valley
Indiana Dunes
Gateway Arch
Petrified Forest
Great Sand Dunes
Everglades
Guadalupe Mountains
Capitol Reef (to be fair, I had lousy weather)
Theodore Roosevelt
Several of these were National Monuments before Congress upgraded them to National Parks--often at the behest of the tourist industry in a particular state. A president can declare any federal land to be a National Monument, but it takes a bill passed by congress to become a National Park.