I usually go to "Easy Tone Ltd."
2887-7082
2882-8763
I can't guarantee they sell at the cheapest price (when compared to Mongkok), but I can pretty much guarantee they are fair and square.
Their prices are much lower than Gome, Fortress, etc. I know for a fact that customers from abroad flock to their shop. And most importantly, they sell brand new products, not refurbished.
One thing you probably want to mention is whether you want a "grey import," or an "official import" phone.
Just in case you are wondering, "grey import" means that a company (not NOKIA, for example) bought stock from another country, and then sells the phones here in Hong Kong. This means NOKIA will not cover any warranty. "Grey imports" are cheaper though.
"Official import" means that the official distributor(s) of the product (NOKIA for example), imports the phones and then distributes them to shops all over Hong Kong for sale. These are slightly pricier, but they do come with official warranty (sometimes including China and Macau aside from HK).
Hope this helps.