Briefly: The tallest building in the world? The Burj Khalifa. Duh. But if plans had worked out differently for other buildings, the Burj Khalifa wouldn't even crack the top 10 tallest buildings in the world. Here's a cool chart showing all the tallest planned buildings in the world. Some of them are still ongoing while others have long been canceled. I wish they all existed in real life.
The chart was made by Martin Vargic, the buildings in black and the 3 ridiculously huge structures in the background were designed and planned but never actually created. The grayed out buildings in the foreground were actually made.