I love Glacier too, for the exact opposite reason I love Zion. You can see mountains all around you in the distance in all directions in Glacier, whereas the main part of Zion is dominated by monoliths right on top of you. The contrast is amazing between the two parks. There is so much to explore in Glacier and so many of the trails are incredibly special. It is impossible to pick a favorite. There is something for everyone in Glacier.
I like the North Cascades. Even though it is so close to Seattle, you can still find a lot of solitude. I think this must be because it isn't much of a drive-through sort of destination; you have to get out of the car and hike in order to see it at its best.
Glacier is by far my most favorite National Park I have visited, with the Grand Tetons a close second. Both parks have areas where there can be crowds, but they also provide some great backcountry spots where it doesn't take much hiking to get away from the majority of the crowds. I've found in both parks that it only takes a little research and a little hiking to find yourself in the midst of beautiful country and able to see some amazing wildlife.
We are taking an old fashion trip to Glacier National Park this summer by Amtrak from Sacramento to East Glacier, then to Chicago. Only there for 3 days and staying at the Swiftcurrent Historical Cabins to view wildlife on the mountains. Should see mountain goats, big horn sheep and hopefully grizzly. Amazing hiking trails especially Grinnell Glacier. When Sperry Chalet gets rebuilt after last years wildfire, that will be another great hike, hopefully next year. It is common to see wildlife wondering the campgrounds and parking lot. They live there. Don't miss the huckelberry ice cream, pie, or milk shakes.
We've been to 11 so far and have found that it just takes a little hiking to get away from the crowds no matter which park we are at. Glacier, Yosemite, Arches, and Great Smoky Mtns are my top parks - they are popular but for a reason! We hit Yosemite, Glacier, and Arches in shoulder months so we avoid the majority of the traffic.
For a little more unknown gems - we have Cumberland Island National Seashore in Georiga that has amazing camping under hanging moss oak tree, steps away from the beach. You have to camp in order to stay overnight so it's extremely quiet after the last ferry leaves for the day. There are wild horses still on the island and overall it's so incredibly beautiful.
We also did Biscayne and Everglades in January and had an amazing trip - those are definitely under rated parks!
I just visited the Wright Brother's National Memorial last week while on vacation and it's well worth the stop and $10 admission fee. So much local history to learn about and they even have life-size replicas and a small airport on site. Definitely worth checking out in the Outer Banks!
@Earthmom, I'm not a fan of crowds either! Here are a few of my favorites.
I love, LOVE, l-o-v-e Great Basin National Park. Got a parking spot at the busiest trailhead on a holiday weekend in the late morning, with plenty of parking spaces to spare.
Crater Lake NP is quieter too. With the Cascades National Recreation Area surrounding it, there is also plenty of gorgeous Forest Service to play on!