Whenever I'm in HK I go check out Tin Cheung Camera (Shop 5 Friends' House, 6B Carnarvon Road, TST). They have both new and used items and I've had the best prices from them. They also carry a lot more specialty items than other stores.
The other two shops I regularly go to for pricing are Man Shing Camera and Wing Shing Camera. They're both in Mong Kok and have the same owner. However, since they have different sales people, you may be able to bargain better at one than the other. As far as I can remember, they only sell new equipment.
For second-hand gear, go to the Champagne Court building (Kimberly Road and Carnarvon Road, TST). The ground floor is packed with second-hand camera stores. As with most things in HK, remember that you can bargain
There are other shops that are good too, but the above are the ones that I would feel confident recommending. However, remember my note regarding grey market, and T1an's note about warranty. Finally, check
http://www.bhphotovideo.com/ to compare prices.
Happy shooting!