Transition metal complex with highest value of crystal field splitting ($\Delta$₀) will be :

Crystal field splitting $\left(\Delta_{0}\right)$ for octahedral complexes depends on oxidation state of the metal as well as to which transition series the metal belongs. For the same oxidation state, the crystal field splitting $\left(\Delta_{0}\right)$ increases as we move from $3 d \rightarrow 4 d \rightarrow 5 d$<br/><br/>$\mathrm{Cr}^{3+}$ and $\mathrm{Fe}^{3+}$ belong to $3 d$ series, $\mathrm{Mo}^{3+}$ belongs to $4 d$ series and $\mathrm{Os}^{3+}$ belongs to $5 d$ series. <br/><br/>Therefore crystal field splitting $\left(\Delta_{0}\right)$ is highest for $\left[\mathrm{Os}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$.