Most houses in big cities are supplied with water from underground or borewell. The water supplied from borewell consists of very high levels of TDS, rust, iron, lead, fluoride, etc.
Hence, the best type of water purifier for this case is RO water purifiers that use reverse osmosis to remove the harmful contaminants through a semi-permeable membrane.
Borewell water also contains high levels of germs and bacteria that cause water-borne diseases. For this, the recommended water purifier is a RO+UV water purifier where the UV lamp is used to kill the disease-causing micro-organisms by attacking their DNA and preventing them from multiplying.