Yellowstone National Park
Yellowstone is the first US National Park I've visited, but I'm sure it's the most beautiful. In the summer, the weather is moderate, and bison are everywhere! I travelled here in with my family and we stayed at a little motel in West Yellowstone, Montana called the Brandin' Iron Inn (not making that up, follow the link). We had a great time and the sites, cliffs, geysers, and wildlife were stunning. We also got to see the famous Old Faithful!