This excellent site gives details of the Peak Vehicle Requirement:
http://www.londonbusroutes.net/details.htm
which you can multiply by capacity of a vehicle and divide by the route
length to get a 'passengers capacity per km' figure. Or multiply the
frequency by the vehicle capacity for 'passengers per hour'. That
should tell you which route has the most capacity. Whether that would
actually be the busiest route, who knows!

Dominic