In the late 1940s, Chicago Midway was the busiest airport in the United States by total aircraft operations – i.e., including every training aircraft practicing take-offs and landings. New York LaGuardia had the most airline operations and passengers until the early 1950s, when Chicago became the busiest airport in the United States by any criterion. Before World War II, Chicago Midway was the origin or destination of one in four U.S. airline flights, although a 1939 Official Aviation Guide shows more airline flights scheduled at Newark than at Chicago.
Gatwick has the world's busiest single-use runway. It does have two runways on site but the close proximity of the two and the length and lack of guidance for the second means that only one is used.