What is the tallest building in the world?
Are we considering whether communication towers with observation galleries should be considered habitable buildings?
Are only habitable heights considered?
Are roof-top antennas should be considered towards height of buildings?
Are components that look like spires classified as antennas or architectural detail?
The answers to these questions greatly determine the final answer to your question. See a complete breakdown of all the world's tallest structures here: http://en.wikipedia.org/wiki/World%27stalleststructures
Dawn's answer, of course, is only partially researched. The Taipei Tower is only the tallest with regards to its spire. If we are counting only inhabitable space, then it is the Shanghai World Financial Center. If we are counting transmission antennae, It is the Sears Tower in Chicago.
When completed, however, the Burj Dubai Tower will put all of them to shame.
the 1 they are building in Saudi arabia I think - apparently if you are standing at the top you can actually see the worlds curve and watch earth spinning around.
Taipei 101 tower in Taipei, Taiwan