Double-decker air conditioned buses are the majority of the public transport in Hong Kong. The characteristic of Hong Kong buses is, most of them are "wearing" colorful cloths --- advertisement is dominantly cover the fleet. For my understanding, double-decker bus is one of the options in western city, most of the buses are single decker in the States. However, double decker is very typical in Hong Kong. Nearly all public buses became double-decker since 15 years ago, before air-conditioned bus is introduced. Tourists may be captured by those beautiful fleet when sightseeing in the city. Those open upper decker buses are interesting, especially when the bus is running fast on the highway, or shuttle between the narrower roads along the mid-levels.
I would like to recommend you route 15C in Hong Kong Island, this route using the bus with "open upper decker". You can enjoy open air whentravel around.