What a great thread. It's hard to pick a favorite, but here goes:
Yellowstone and Death Valley.
Yellowstone for the sheer scale of it, both in the magnitude of the sights and their diversity. The park is enormous and takes hours to drive across and seemingly has everything: towering waterfalls, thermal features of all varieties, mountains, meadows, wildlife, canyons, an enormous lake, and no matter how crowded it seems it feels like you can always get away to your own less-traveled area for a hike.
Death Valley surprised me for its quiet beauty. It's accessible, just a few hours' drive from Las Vegas, it's also huge: you feel so small when standing at the bottom of the valley, looking up at the 'sea level' sign high up on the cliff. The sunrises and sunsets are remarkable.
I've never been to a park I didn't like, but if I were to recommend parks for people to visit, I'd put those two parks at the top.
One day I hope to have visited them all. Next up are Guadalupe Mountains, Big Bend, and Carlsbad.
Visited: Acadia, Shenandoah, Great Smokey Mountains, Biscayne, Everglades, Congaree, Badlands, Painted Desert/Petrified Forest, Grand Canyon, Saguaro, Joshua Tree, Channel Islands, Redwood, Lassen Volcanic, Death Valley, Yosemite, Great Basin, Zion, Bryce Canyon, Capitol Reef, Canyonlands, Arches, Black Canyon of the Gunnison, Mesa Verde, Great Sand Dunes, Rocky Mountain, Grand Teton, Yellowstone, Glacier, Crater Lake, North Cascades, Olympic, Mount Rainier, Kenai Fjords, Haleakala, and Hawaii Volcanos.
Still to go: Guadalupe Mountains, Big Bend, Carlsbad Caverns, Kings Canyon/Sequoia, Pinnacles, Lake Clark, Wrangell-St Elias, Glacier Bay, Katmai, Kobuk Valley, Gates of the Arctic, Denali, Wind Cave, Theodore Roosevelt, Voyageurs, Isle Royale, Cuyahoga Valley, Mammoth Cave, Hot Springs, USVI, Dry Tortugas