Great Smoky Mountains National Park, U.S.
Located in North Carolina and Tennessee, this large national park contains a section of the Appalachian Trail. Though it does not have as much notoriety as parks like Yellowstone, Yosemite or the Grand Canyon, the Smokies get more visitors than any other national park, by a very large margin. In 2012, more than 9 million people came here. That was more than visited the Grand Canyon and Yellowstone combined (4.4 million and 3.4 million, respectively).
By some estimates, more than 400,000 hikers use the park's 800 miles of trails each year. The tower at Clingmans Dome and summit of Mount Le Conte are crowded during the summer because of their amazing views, and the historic site at Cades Cove sees more visitors than any other part of the park. Like all popular national parks in the U.S., Smoky Mountains has another redeeming trait: You can step outside the popular sites and explore the backcountry in almost complete solitude.