Disinfecting action of chlorine in chlorination of water is mainly due to:
Hypochlorite ion
Chloride ion
Sodium chloride
Hypochlorous acid
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