If you are looking to visit one of the tallest buildings in the world or, for that matter, experience one of the top ten observation decks in the world, you might want to start looking outside the USA.  The Willis Tower in Chicago, IL currently ranks as the 9th tallest observation deck in the world and the only high rise in the United States in the top ten.  However, it won't stay this way for very much longer.  By 2020 the Willis Tower is expected to fall all the way to #17, and #18  by 2021.