Disinfecting action of chlorine in chlorination of water is mainly due to:
Ref. Foundations of Community medicine. Page. 62
 
Chlorination

The disinfecting action of chlorine is predominantly due to hypochlorous acid.
Hypochlorous acid is most effective form of chlorine and it is almost 70-80 times more effective
Hypochlorite ion
Chloride ion
Sodium chloride
Hypochlorous acid