Great topic.
Parks I've been to:
1. Denali N.P. I was a park ranger there. Incredible place. Sub-arctic wilderness.
2. Joshua Tree. Began going here on spring break trips down to SoCal to visit my mom, who lives about 1.5 hrs. away. Much fun with my son, camping and scrambling the amazing, Jumbo Rocks. Beautiful place.
3. North Cascades. My son and I did a five day kayak-camping trip on Ross Lake(not really part of the park, more of a Nat'l. Recreation area, but surrounded by the park).
Now, gotta go back and hike, etc. there. Very rugged. Maybe the Alps of North America. Definitely one of the less-visited national parks.
4. Olympic. great time touring, mostly, the peninsula. We did hike down to Shi Shi beach for an overnight, which was lovely. Only hit the tip o' the iceberg with this place, as the backcountry beckons.
5. Grand Teton. Very fun place. Hiked, and one stormy day, kayaked a ways down the Snake river, with views of moose and the clearing Tetons.
One of the best days of my life. Wanna go back and climb the Grand one of these days.
6. Yellowstone. At the time, (right after the Tetons), my son proclaimed, "this is definitely my favorite national park." Amazing place of scenic wonders. Would love to go back and actually backpack, and/or kayak on Yellowstone lake.
7. Glacier. A gem. Reminiscent of the Canadian Rockies. The purity of the place, even with the tourists, is something that found it's way into my soul. Wanna go back and climb some of those mountains.
8. Yosemite. Awesome beauty as well, but I've really only briefly explored. Hope to make it this summer's trip, and play in Tuolumne, and scramble some of those peaks.
9. Mt. Rainier. Son and I did a 3 day backpack on one of the highest, prettiest sections of the Wonderland trail. Fantastic. Massive mountain. 28 glaciers on her. Wildflowers galore.
In Canada, Banff and Jasper. Back to the purity of the northern Rockies which I mentioned regarding Glacier. First time I experienced them was on a solo bike trip, pedaling every day through that stunning, breathtaking beauty. Canadian Rockies, certainly on of my favorite places. Need to go back.
That's about it for now. . .
A couple of notes on what others have mentioned. Yep, they are crowded, and for good reason. Unique places, all of them. Each one very different. Hard to pick a fave; I appreciate them all for being so unique and wonderful in their own right.
Read one time when I was working in Denali, that something like 90 to 95% of all visitors to national parks, never leave the main thoroughfare, whether that be Going to the Sun Road, Denali park highway, the overlook at Grand Canyon, or the Valley floor of Yosemite.
So, wanna ditch the crowds? Sounds like many of you already know how: just put on your walkin' shoes/boots, and get out there. You'll be one of the 5-10% who see the "real park," away from the buses and ice cream and souvenir shops. Though, my son and I do stop at a couple of those places; we just do it on our way out of the park.