The element with highest standard reduction potential (in volt) $\left[\mathrm{M}^{2+} \rightarrow M\right]$ among the first row of transition elements is

In the electrochemical series, $\mathrm{Cu}$ is below hydrogen. Hence, it has positive value of standard reduction potential $\left(E_{M^{2+} / M}\right)$. The standard reduction potentials are $$ \begin{gathered} \mathrm{Ni}(Z=28) \Rightarrow-0.257 \mathrm{~V} \\ \mathrm{Cu}(Z=29) \Rightarrow+0.337 \mathrm{~V} \\ \operatorname{Cr}(Z=24) \Rightarrow-0.90 \mathrm{~V} \\ \operatorname{Ti}(Z=21) \Rightarrow-1.63 \mathrm{~V} \end{gathered} $$ Highest reduction potential is of $\mathrm{Cu}$ as, it has positive value of reduction potential, while $\mathrm{Ni}, \mathrm{Cr}$ and Ti have negative reduction potential value.