Mesa Verde, Arches, Bryce.
Canada: Jasper, Banff
Mesa Verde One of the most unique parks I've ever seen.. Near Durango, which is fun too. Then drive to Moab, Canyonlands, Arches.
These desert parks are really fragile, so please stay on the trails. Last few years the mobs have pummelled some of the fragile plants. They grow on a lifecycle of decades.
Arches, go to Moab airport and rent an airplane. You will see Arches from angles that are just mindblowing. Also I had the pilot fly into the canyons and saw 900 year old ruins that were 1000 fet off the bottom of the canyon. Can't fly into the park, but that's not a problem, as these things (ruins, arches, etc) are outside the park too.
I still love Yellowstone and Glacier though.
Any park: If you want to avoid the hordes do the dawn/dusk thing just like hunting. Tourists don't get active until about 9AM. Also if you want to be alone, go more than a mile from pavement.
Sometimes I like to be in the crowds though, I've seen tourists do stuff that has me convinced I'm going to get viral video of some Darwin reject going over the head of a bison. Kinda hoping to see some natural selection at work.