Which aluminium alloy is used in the Boeing 747's fuselage?
The primary alloy for the original 747 is 2024-T3, which is not the strongest or otherwise perfect, but a well-established convenient choice. Second is 7075-T6, used for major load-bearing parts. 

7075 is stronger, but less workable and less tough than most; it comes closer to the edge for aluminum, so to speak. 
6061 follows for for less-loaded or some more complex parts, where it's a staple, being overall the easiest to use out of common Al alloys. This is not nearly an exhaustive list, just the main alloys.