And you thought it was some giant building in Dubai. Nope. According to measurements converted from in-game models ad referenced for scale, City 17's own Citadel, constructed by the Combine, easily takes the cake.
Standing at 8430 feet tall, the Citadel easily, ahem, towers over the competition, Dubai's Burj Khalifa only managing to get 2723 feet off the ground (note: the image above is in metres).
Before anyone complains about the tense used in this post, let's assume there are people here who haven't played Half-Life 2's episodes.
The scale of Half Life's Citadel, compared to the tallest buildings in the world [Reddit]