Fairly recent news... Barcelona-Madrid now slips into first place as busiest route (movements).

Source:
http://www.oag.com/oag/website/com/OAG+ ... tes+070906

Top 3 busiest routes 2006:

1. Madrid-Barcelona (Spain)
2. Sao Paulo-Rio de Janeiro (Brazil)
3. Sydney-Melbourne (Australia)

Top 3 LCC routes 2006:

1. Dallas-Houston (US)
2. Sydney-Melbourne (Australia)
3. Denver-Las Vegas (US)