Trying to do this, I realize how many great places my wife and I have seen. VERY difficult. however, here's my best attempt:
1) Grand Canyon (especially hiking down Bright Angel Trail)
2) Smoky Mountains National Park
3) Arches
4) Bryce Canyon
5) Yellowstone
I like each of these for different reasons, but want to return to all of them as well as others such as Canyonlands, Grand Tetons, Yosemite, etc. Plus this doesn't count places we're planning to see such as Rocky Mountain National Park, Glacier NP and Capitol Reef NP.