Yellowstone is not only the first national park in the United States, but the first national park in the world. Created in 1872, Yellowstone has held its position as the greatest of the parks in North America.
What makes Yellowstone so special is a combination of the geothermal activity which accounts for the hot springs and geysers, the landscape which creates such spectacles as the Grand Canyon of the Yellowstone, and the large number of megafauna which inhabit the park.
Its uniqueness was recognized when it was one of the first 12 sites to be declared a UNESCO World Heritage Site in 1978.
Yellowstone is one of the most difficult to get to parks in the continental United States as there are no major cities in close proximity. The closest airport is in West Yellowstone, MT but flights to Jackson Hole, Bozeman and Boise can also get you to the park with some driving. The best option is to have your own car as the park is very large. Plan to spend at least 3 days to see the park properly.
Yellowstone is one of the few places which I would never refuse a return visit. I hope to make a future visit in the winter to photograph the park covered in ice and snow.