Strongest oxidant among the following is

$\mathrm{Mn}{{\mathrm{O}}_4}^{-}$ has the highest oxidation state

$\mathrm{Mn}$ in $\mathrm{Mn}{{\mathrm{O}}_4}^{-}$ is +7

$\mathrm{Mn}{{\mathrm{O}}_4}^{-}$ is the best oxidizing agent

Electronegativity increases when oxidation state increases.

The highest oxidation state is considered as +7, or even +8.