I’ve been to a handful of NPs but have to say Yellowstone was my favorite! The fact that part of BTMRR is modeled on Mammoth Hot springs was icing on the cake, lol.
In fact, I just got back from a big summer road trip a couple weeks ago - we went to Theodore Roosevelt, Yellowstone, Grand Teton, Arches, and Rocky Mountain. Yellowstone was my fav, but TR was a very pleasant surprise. I liked it better than the Badlands in SD.
I have also been to Great Smoky Mountains several times and love it, but I love that whole area, not just the NP.
And I refuse to consider the Gateway Arch in St Louis to be a NP (no offense to anyone from MO)! I appreciate what it stands for, I just think it should be a National Monument because it is man made.