Which is the strongest oxidizing agent of the following?

$\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$

$\mathrm{CuSO}_{4}$

$\mathrm{KMnO}_{4}$

$\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}$