1. Burj Khalifa At the Top — Downtown Dubai
Observation decks on levels 124, 125 and, for the SKY ticket, 148.
The official site describes the At the Top observatory at 456 meters on levels 124 and 125, and At the Top SKY adds level 148 at 555 meters, which it calls the highest deck in the world. The ride up includes projections, and level 125 has a falcon's-eye-view of the city.
Sunset is the most popular slot; go for it if the weather is clear, and compare it against a morning slot if you want fewer people and cooler light.
Good to know: Book a timed ticket online, and choose a clear-day slot: haze can hide the view in summer.
1 Mohammed Bin Rashid Boulevard, Downtown Dubai, Dubai, UAE