69

Note the number 69 on the runway

The 69 sex position is an innuendo recurring throughout the GTA series. This page lists some of its appearances:

The game Grand Theft Auto: London 1969 (although this may be incidental).
Runways at all airports in the GTA III Era being numbered 69.
The play 69th Street advertised in Grand Theft Auto III.
Area 69 military base in San Andreas.
Shipping containers bearing the number 69.
69 Cents used as prices and the name of a chain of discount stores.
The San Fierro 69ers American Football team, also a reference to the San Francisco 49ers.
Pier 69 in San Fierro, a reference to Pier 39 in San Francisco.
On the side of the aircraft carrier in Easter Basin Naval Station in San Fierro.
The mission in Grand Theft Auto: Liberty City Stories - Crazy '69'.
Officer '69', the generic police officer and a multi-player character of Grand Theft Auto: Liberty City Stories.
Marquis in Grand Theft Auto: Vice City Stories had "Marquis 69" written on the rear of the boat.
The Fire Trucks in Grand Theft Auto IV have the numbers 69 written on the side of them.
The 69th Street Diner in Grand Theft Auto IV. Similar-looking "60 Diner" restaurants are also seen around the city.
On the side of buses in Liberty City.
On the Bloodring Bangers in Vice City.
The achievement Get Good Wood in The Lost and Damned is earned by knocking 69 bikers off of their bikes with a bat.