All the following species are strong oxidizing agents. Their...

The standard reduction potentials of the above species in acidic medium are as given below
${S}_{2}{O}_{8}^{2 -} ( aq ) + 2e^{-} \to 2S{O}_{4}^{2 -} ( aq ) ; E ° = + 2.01\text{V}$
$Mn{O}_{4}^{-} ( aq ) + 8H^{+} + 5e^{-} \to Mn^{2 +} ( aq ) + 4{H}_{2}O; E ° = + 1.77\text{V}$
$C{r}_{2}{O}_{7}^{2 -} ( aq ) + 14H^{+} + 6e^{-} \to 2Cr^{3 +} + 7{H}_{2}O; E ° = + 1.33\text{V}$
The more the standard reduction potential, the more the tendency of the species to undergo reduction and more strongly it (species) would work as oxidizing agent.
As a result, their strength as oxidizing agent in acidic solution decreases in the following order
${S}_{2}{O}_{8}^{2 -} > Mn{O}_{4}^{-} > C{r}_{2}{O}_{7}^{2 -}$