Which complex compound is most stable?

Chelating ligands in general form more stable complexes than their monodentate analogs. When a chelating ligand binds to a metal ion, it forms a ring structure with the metal ion at the center. This ring structure increases the stability of the complex because it restricts the movement of the metal ion and prevents it from interacting with other molecules or ions. In addition, the chelating ligand can donate multiple electron pairs to the metal ion, which increases the strength of the metal-ligand bond and further stabilizes the complex.

∴ The most stable complex is $\left[{CoCl}_{2}{\left(en\right)}_{2}\right]{NO}_{3}$