I'm cheating a little because it's not actually my favorite, but if you're ever in the Midlands of South Carolina, definitely check out Congaree National Park. It's not the largest or the most beautiful, but it's very different from anything you've probably seen before. It's technically a "bottomland hardwood forest" -- I would call it a swamp -- and there is very little of that biome left in the US. Much of the park is a floodplain with endless cypress trees with their little knees sticking up out of the water. There's a boardwalk near the visitor's center because it's almost always too wet to walk through that area, but even the boardwalk floods when it rains a lot. It's perfect if you want a break from the breathtaking mountains, valleys, and canyons that dominate many of the National Parks.