If Half-Life's Citadel actually existed, it would dwarf the world's tallest buildings.
Dubai's Burj Khalifa, the record-holder since 2010, measures 2,723 feet tall. According to calculations done using Valve's Hammer Editor, the Citadel is a whopping 8430.9 feet tall.
In case you're wondering about the math on the chart above (which shows the tower at 11241.2 feet), it's based on old information about the conversion between in-game "Hammer units" and real-life measurements.
Mass Effect's Citadel would be even taller, by the way. Its central Presidium Tower measures 3,435 feet.