Interesting study by IATA on the world's busiest passenger routes in 2013. 

1. Hong Kong to Taipei -- 4,876,595
2. Dublin to London -- 3,562,951
3. Jakarta to Singapore -- 3,435,375
4. Seoul to Tokyo -- 3,272,375
5. London to New York -- 2,729,362
6. Amsterdam to London -- 2,724,022
7. London to Milan -- 2,075,485
8. Honolulu to Tokyo -- 1,895,611
9. New York to Toronto -- 1,600,226
10. New York to Paris -- 1,545,938