1. Willis Tower
It might have lost the world’s tallest building slot to Taipei 101 Tower in Taiwan, thanks to its enormous spire, but Willis Tower, formerly known as Sears Tower, is still the tallest in the United States, at 1,450 ft. Designed by Chicago firm Skidmore, Owings & Merrill, the tower uses nine exterior frame tubes, avoiding the need for interior supports. For awesome 360-degree views of the city, head to the 103rd-floor Skydeck. From here you will also be able to access The Ledge, a series of enclosed glass boxes jutting from the Skydeck and providing a fascinating view straight down to the ground.