The couples $\text{Mg}^{2+}/\text{Mg}$ ($-2.37\ \text{V}$), $\text{Cu}^{2+}/\text{Cu}$ ($+0.34\ \text{V}$), $\text{Ag}^+/\text{Ag}$ ($+0.80\ \text{V}$) and $\text{Fe}^{3+}/\text{Fe}^{2+}$ ($+0.77\ \text{V}$) are given. Which ion is the strongest oxidising agent?