I recall (at least from 6 months ago) that at Chungking Mansions, one of the first ones on the left just inside the entrance (maybe Western Union affiliated but don't fully recall) actually gave the best rate (for CNY-HKD-USD) after looking around 3 different shops. Sometimes, the first one turns out to be the best! He only had a small calculator producing a paper receipt with the proposed rate, no fancy computer or anything.