Hong Kong water quality is not all that terrible - there is very little if any organic matter - i.e. bacteria in the water - biggest issue is chlorine and other chemicals and metals.
So what you want is something that takes out the chemicals , metals ( due old pipes ) and any organic matter ( unlikely but possible) that may be still in the water.
Best - and by far most cost effective - is a ceramic system. Don't waste your time with complex pressured or revers osmosis systems.
Get a Fairey/ Doulton setup. Simple and easy to install. Water tastes great. Environmentally friendly filters too.
Drinking water treatment technologies,water purification methods