I'd say Yellowstone also. I've been to Yellowstone, the Grand Tetons, Denali, Arches, Canyonlands, Big Bend, Grand Canyon, Badlands, Wind Caves (I think that's a NP), can't remember any others. I'd like to get to Arcadia some day.
Just like my vacations, the last one is often the favorite because it is freshest in my mind. The last national park we visited was Death Valley. It was a warm 120 degrees the day of our visit. There is so much to see and the park is so different based on the season you visit.
My favorite so far is the Great Smoky Mountains, but I haven't been to enough of them to really know for sure. We're planning a several month trip to tour many of them very soon so I'd imagine my opinion may change.
I went to two national parks in the last few weeks. Crater Lake and Mt Hood National Forest. I really want to buy a cabin in Mt Hood, so probably that one, because I can easily get to it and enjoy when I want.
I couldn’t even pick. They are all so awesome. But I have to say the ones in Utah are pretty spectacular. Like another world. While Arches, Zion and Bryce get the acclaim, I was also amazed at Canyonlands. The colors of the vistas that stretch on for miles is just incredible.
Shenandoah, hands down. But I've only been to a handful of the parks so far. I think we have 15 national parks in our summer road trip itinerary, so I might have a new favorite by the time we get home.