Did Death Valley 2 years ago - way underrated, bring gallons of water. Also did Red Rocks (technically a state park) and Valley of Fire that trip.
Yellowstone last year - everything else kind of boring after that.
I've been on vacation the last week - did Gettysburg, Shenandoah and DC. Going home tomorrow - sucks.
Done Acadia in the past too.
Each one unique, guess it depends what you want - hikes (Shenandoah and Acadia - some are more rock scrambles than hikes), history (Gettysburg/DC monuments), natural wonders and wildlife (Yellowstone), endurance (Death Valley).
Yellowstone is a must do - huge, diverse, full of wildlife - geysers, thermal pools, canyons, waterfalls. Bison, elk, deer, bears all over. Grand Teton not far too.
Next up on my wish list are Yosemite, Bryce and Zion.