Given that,

${\mathrm{E}}_{{\mathrm{O}}_2/{\mathrm{H}}_2\mathrm{O}}^{\mathrm{o}}=+1.23 \mathrm{V};$
${\mathrm{E}}_{{\mathrm{S}}_2{\mathrm{O}}_8^{2-}/{\mathrm{SO}}_4^{2-}}^{\mathrm{o}}=2.05 \mathrm{V}$
${\mathrm{E}}_{{\mathrm{Br}}_2/{\mathrm{Br}}^{-}}^{\mathrm{o}}=+1.09 \mathrm{V};$
${\mathrm{E}}_{{\mathrm{Au}}^{3+}/\mathrm{Au}}^{\mathrm{o}}=1.4 \mathrm{V}$

The strongest oxidizing agent is

For strongest oxidizing agent, the standard reduction potential should be highest.

An oxidizing agent, or oxidising agent is a substance that has the ability to oxidize other substances — in other words to accept their electrons. Common oxidizing agents are oxygen, hydrogen peroxide and the halogens.