Lithium is the best reducing agent in aqueous solutions. This is best explained by

$$\begin{gathered} M_{\left(s\right)}\stackrel{+\Delta H_{Sub}}{\to }{M}_{\left(g\right)} \\ {M}_{\left(g\right)}\stackrel{+\Delta H_{IE}}{\to }{M}_{\left(g\right)}^{+} \\ {M}_{\left(g\right)}^{+}\stackrel{-\Delta H_{Hyd}}{\to }{M}_{\left(aq\right)}^{+} \\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ {M}_{\left(s\right)}\stackrel{ }{\to }{M}_{\left(aq\right)}^{+} \Delta H=\Delta H_{sub}+\Delta H_{IE}+\Delta H_{Hyd} \\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \end{gathered}$$

Greater the negative value of net 'ΔH’ value greater the feasibility of the process. $L{i}^{+}$ has abnormally high hydration enthalpy.

∴ net ‘ΔH’ is most negative for Lithium.

$\therefore L{i}_{\left(s\right)}\to L{i^{+}}_{\left(aq\right)} is most feasible.$

Thus lithium undergoes oxidation most readily and it is the best reducing agent in aqueous solution.