Out of F2,Cl2, Br2, and I2, most oxidising agent is

F2 can oxidise Cl-, Br-,I- to Cl2,Br2,l2.Cl2 can oxidise Br−,I−to Br2,I2.  Cl2 cannot oxidise F− Br2 can oxidise I−to I2.  Br2 cannot oxidise Cl−,F−.  Thus, oxidising power is F2>Cl2>Br2>I2