Love the National Parks. So far would have to rate them (these are the only ones I have seen):
Glacier - Saw grizzly, moose. Spectacular scenery at relatively low elevation (8,000 to 9,000 feet to the summits). The highway takes you into the bowels of the great scenery. Very interesting architecture in the huge lodges, beautiful lakes. Glaciers. Cool weather in August.
Yellowstone (VERY close second) - saw black bear, bald eagle, bison, etc. Very large and contains very interesting geothermal attractions, beautiful mountainous scenery at high elevations, awesome water falls and yellowstone lake is very scenic. Cool weather in August. Love it.
Rocky Mountain - trail ridge road - about 20-25 miles above 12,000 feet - accessible high meadow conditions, wildlife, 14,000 footers all around. Glaciers....very scenic. Love it.
Mesa Verde - very scenic, large park with lots of indian ruins, some of which have been restored. Cool weather in August. Love it.
Carlsbad - incredible huge caverns in the middle of a semi-arid desert. Cool in the caverns, warm and dry otherwise, not uncomfortable. Very enjoyable visit though a long ways from anywhere.
Grand Canyon - only have seen the north rim and after a half hour or so at the park it came a big storm so didn't get to stay long. Would not consider going down in it - too dang hot in August although comfortable up on the rim. Very awesome natural formation but jI can't see spending alot of time there unless you want to see it very many different angles or take a trek down to the bottom.
Smokey Mountain - very scenic but also very crowded, pretty cool up around 6,000 feet. Would have enjoyed it more if not for several 2-3 hour traffic jams we got in to. Probably won't go back, just too far east and too many folks.
Big Bend - very scenic and has remote areas where it is a little scary - no other people to be seen. A little warm in July and August.
Have been to Mt Rushmore and it is very interesting and the black hills are scenic. Not a national park I don't think though. Ditto on Devils Tower in Wyoming. Worth the visit though a long ways from anywhere. Glad I saw both, but they don't compare with the top 3 national parks as a destination.
Bucket List:
Going back to Glacier - this time will make it into Canada. Got our passports 4 years ago but in the 4 or 5 days at Glacier we didn't get done checking out the U.S. side. At nearly 60 still have never been out of the states.
Redwood in California
Rainer, Olympic, and North Cascades in Washington.
Yosemite, but I heard it can be very crowed, and hot in the summer. I leave Texas to escape the heat. Have not been there so I have no first hand knowledge.