Yellowstone invented the national park, and it is still the wildest show in the system. The park sits atop an active supervolcano whose heat drives more geothermal features than any other place on Earth, over 10,000 geysers, hot springs, mud pots, and fumaroles. Old Faithful earns its fame. Grand Prismatic Spring, a steaming lake ringed in orange, gold, and impossible blue, is the one that ends up printed large on your wall.
The other half of the park is the American Serengeti. Bison herds stop traffic in Hayden Valley, wolves hunt the Lamar Valley at first light, and grizzlies work the meadows between. Go at dawn with binoculars and patience. Two hours in the Lamar before breakfast beats a full day of boardwalks, and we say that with real affection for the boardwalks.