Which of the following cation is the best oxidising agent?

${\mathrm{E}}_{{\mathrm{Zn}}^{2+}/\mathrm{Zn}}^0=-0.76 \mathrm{V}$

${\mathrm{E}}_{{\mathrm{Cu}}^{2+}/\mathrm{Cu}}^0=+0.34 \mathrm{V}$

${\mathrm{E}}_{{\mathrm{Ag}}^{+}/\mathrm{Ag}}^0=+0.80 \mathrm{V}$

${\mathrm{E}}_{{\mathrm{Cr}}^{3+}/\mathrm{Cr}}^0=-0.74 \mathrm{V}$

It is because ${\mathrm{E}}_{{\mathrm{Ag}}^{+}/\mathrm{Ag}}^0$ has the highest value.   Oxidising power is directly proportional to SRP values.

Highest positive value indicates greatest tendency to convert from +n to zero state (greatest tendency to undergo reduction).