What is the fastest way from Hong Kong to Dongguan?

You can take direct long-distance bus at Metropark Hotel Causeway Bay Hong Kong.
There are several get-off stations in DG downtown area. You can choose the one that is near to your destination.
The schedules are from 06:50 to 19:50 with the interval of an hour. The duration is 80 to 125 mins. :-)