Compare the stability of +2 oxidation state for the elements of the first transition series.

In the beginning of 3d transition series, Sc²⁺ is virtually not known or in other words it is not stable in comparison to Sc³⁺, Ti²⁺, V²⁺, Cr²⁺ are known but less stable in comparison to their most common oxidation state of +3.
In the middle Mn²⁺, Fe²⁺, Co²⁺ are known and quite common. In fact Mn²⁺ and Mn⁷⁺ are most stable in Mn. Fe²⁺ is less stable in comparison to Fe³⁺ but is due to fact that Fe³⁺tends to loose one electron to aquire d⁵ structure which has extra stability. Co²⁺ is less stable as compared to Co³⁺. Ni²⁺ is most common and most stable among its +2, +3, +4 states. Cu²⁺ is more stable and is most common species as compared to Cu¹⁺. At end Zn forms only Zn²⁺ which is highly stable as it has 3d¹⁰ structure.