What is the best and most efficient way to get to the Hilton Garden Inn at Mongkok from the HKIA?

You can take bus no.E21 at the Airport Ground Transportation Center and get off at Mong Kok Market Bus Stop. The ticket fare is HK$14/person. Upon arrival, you may walk about 100 meters along Argyle Street and turn left to Ferry Street for around 280 meters. Then you may find your hotel directly. If you plan to stay there for couple days and you may use the public transportation frequently, it will be better to purchase an Standard Octopus Card. If not, then you can buy the ticket directly.