There are a number of ways to rank the busiest airports. You can look at total aircraft movements (takeoffs and landings). You can look at total available seats (because a whole lot of small regional jets may skew those results). Or – the most common measure – you can look at total passengers.
It was really exciting when Atlanta airport crossed 100 million passengers for the year, even if they faked the 100 millionth passenger.
Some cities like London and New York have multiple major airports in their metropolitan areas. In fact, London is the single largest passenger traffic market if you combine the region’s airports and that’s followed by New York and then Tokyo and Paris.