The Financial Times ranks the 20 most popular airline routes in the world.
Hong Kong to/from Taipei is the most popular with 2,138,484 passengers annually.
18th on the list is Atlanta to/from New York with 935,265 passengers annually.
(H/T: TreeHugger - the FT list was published in January, but TreeHugger drew my attention to it today as part of its series on 'greener' flying.)