Currency conversion is an essential “to do” for international travel, and getting the best rates can make a difference in travel costs. In Hong Kong, the most important rule is to avoid exchanging money at the airport as much as possible, as the rates there are among the worst in the entire city. If you have absolutely zero or very few Hong Kong dollars, only exchange enough for your immediate transportation and survival needs, until you can visit a better money changer in town. I also recommend that you get one of the tourist products from the MTR (mass transit) that provides unlimited travel on the system, so there will be no additional transportation costs for visiting an in-town money changer. The “Airport Express Travel Pass” is especially worth looking into, and there may be a discount for purchasing that online or onboard a Cathay Pacific flight.
In-town, the best currency exchange rates in Hong Kong for cash can be found, in my experience, at Ngau Kee Money Changer (牛記找換) located at 1 Cleverly Street in Sheung Wan, between Connaught Road Central and Des Voeux Road Central. From Sheung Wan MTR station take exit B or C, or take the tram to Western Market (if you do not have an MTR unlimited travel pass). Indeed, that section of Cleverly Street is filled of money changers, so feel free to compare rates. However, my experience is that no one offers a better exchange rate than Ngau Kee.
(“Ngau Kee” refers to a bull. With a rich agricultural heritage, “bull” is a common traditional Chinese name for men, as in “he is a strong as a bull”.)