Been to: Redwood, Glacier, Great Basin, Arches, Canyonlands, Mesa Verde, Bryce Canyon, Grand Canyon, Badlands, Wind Cave, Yellowstone, Grand Teton
Glacier is probably my favorite - it has incredible scenery and plenty of hiking. Canyonlands is definitely up there, too - it's the most /out/ of the parks I have been to, in that backpacking is required to get to many of the best parts. Arches and Yellowstone are gorgeous, but full of normies. It's worth going to Yellowstone if you can get there during the winter. Backpacking across the Grand Canyon is amazing, but if you just want scenery, you're better off with some of the other parks in the four corners area.
I hope to get to some of the parks in Alaska, although they aren't as easily accessible as parks in the lower 48. Wrangell-St. Elias and Denali (it's Mt. McKinley, reeee) both look pretty good.