The recently opened Burj Khalifa in Dubai is the tallest building in the world, with the highest occupied floor of any building in the world, and more stories than any building in the world. Of course, Dubai declared bankruptcy last year, which makes this grand opening look less like an accomplishment, and more like an awkward answer to the question: Where did all the money go?
In any case, it's very tall. If you had stacked the WTC twin towers on top of each other, they would have reached only 13 feet higher than the Dubai tower.
This graph from the New York Times is pretty stunning: