The Singapore-Kuala Lumpur route is the world’s busiest air route in the year thru Feb, 2018 according to a recent study. Airlines made 30,537  trips average about 84 trips per day. The flight time between the cities is about an hour.

The World’s Busiest Air Routes are shown in the chart below:

Click to enlarge

Source: Kuala Lumpur-Singapore named busiest international air route, The BBC