Hong Kong Airport at Chek Lap Kok has probably the best public transport links in the World and - thanks to recent fleet replacement programmes - the newest buses serving it. The principal operators are Citybus and Long Win Bus (a subsidiary of the giant KMB company); franchised bus routes to the Airport are more or less evenly divided between the two companies.
Apart from a smattering of Volvo B9TLs, all the buses in this video are products of Alexander Dennis or its precursors. By the time of filming (March 2017), the once impressive fleet of Duple-Metsec-bodied Dennis Tridents on Citybus' Airport services had been whittled down to a mere handful of vehicles, and I only managed to see one of them. All of Long Win's fleet of ALX500-bodied Dennis Tridents have now been retired.
The Airport bus routes fall into three categories:
A - Airport premium services, connecting Terminals 1 and 2 with the most popular destinations, and aimed squarely at visitors.
E - External routes - services to other parts of Hong Kong, which may also serve Tung Chung and the Asia World Expo.
S - Shuttle services, short routes connecting Tung Chung with different parts of the Airport.
The only exception to this numbering scheme is service X1, which shuttles directly between Asia World Expo and Tung Chung Station.