Is a taxi the best way to get from Hong Kong Airport to Metropark Hotel by the Causeway?

Well, you can take the cab from HKIA to your hotel directly. It would cost around 350 HKD for the single trip.
Or upon landing, you can take bus no.E11 and get off at Tin Hau Station. The trip takes around 80 mins. And the ticket fare is 21 HKD per person. Then just walk south around 200 yards to find your hotel.
Choose the transport you like. :)