Open Rice
16 seats - life changing info on the mini bus system!
Not actually an app (opps) just a site 16seats.net
My top 5 are all related to getting around HK:
1) KMB & Long Win bus (bus arrival times. Citybus has an app but no ETA)
2) MTR Mobile (immediate news if there is a delay on the MTR line)
3) Next Train (get arrival times of 4 MTR lines)
4) PokeGuide (find the best door of your MTR train to get you right up the escalator to your desired exit)
5) GovHK Notifications (choose to get Weather/Traffic/Beach&Swimming Pool/Air Pollution {and lots more} real-time updates)
The app I use most that is not already mentioned is MapswithMe - which enables you do use maps without a data connection. So I download all the places I travel and then once there i don't have to run up big fees finding out how to get around.
Other apps I use a lot already mentioned above:
geoexpat
various games
KMB bus app with real-time bus info
cathay
agoda
uber