The oxidising ability of perhalates are in the order:

More the reduction potential value, more is the oxidising power.The reduction potential of BrO4-,IO4- and ClO4-   are 1.85 V, 1.65 V and 1.20 V respectively. Thus BrO4- has the the highest oxidising power and ClO4- has the lowest oxidising power among the given perhalates.