The standard potentials at 298 K are

$E^{\circ }\left({\mathrm{Zn}}^{2+}\mid \mathrm{Zn}\right)=-0.763\mathrm{V},E^{\circ }\left({\mathrm{Cr}}^{3+}\mid \mathrm{Cr}\right)=-0.41\mathrm{V}$

$E^{\circ }\left({\mathrm{H}}^{+}\left|{\mathrm{H}}_2\right|\mathrm{Pt}\right)=0.00$   and  $E^{\circ }\left({\mathrm{Fe}}^{3+},{\mathrm{Fe}}^{2+}\mid \mathrm{Pt}\right)=0.770\mathrm{V}$

Which is the strongest reducing agent?

The smaller the value of reduction potential, lesser the reduction tendency of the ion and larger the oxidation tendency of the corresponding metal. Larger oxidation tendency implies stronger reducing agent.

A reducing agent is a substance that undergoes oxidation to a higher valency state in a redox reaction and loses electrons to other substances in the process. In an oxidation-reduction process, a reducing agent is one of the reactants that reduces another reactant by discharging electrons onto it.