I visit Hong Kong about 10 trips per year, most of the time I will stay in Royal Plaza Mong Kok, rooms and facilities are good. Location is also good, its beside KCR station, walking distance to Prince Edward or Mong Kok MTR stations, 10 mins taxi ride to China Hong Kong Ferry Terminal. Its rated 4 stars iirc.
There was also once when I forgotten to book my hotel, I ended up in a back packers hostel(USA Hostel) in Tsim Sha Tsui, the room is :sweatsm::sweatsm::sweatsm: