Zinc has the lowest enthalpy of atomisation in the first transition series because:

Strength of metallic bonding in transition metals is governed largely by the number of unpaired d-electrons that can participate in delocalised bonding. Zn has the configuration $[\mathrm{Ar}]3d^{10}\,4s^2$ — all d-electrons paired and unavailable. Only the 4s electrons contribute to metallic bonding, giving Zn the weakest interatomic bonding of the first series and hence the lowest enthalpy of atomisation ($\approx 126\,\text{kJ mol}^{-1}$).