Amongst the following, the most stable complex is :

Stability order : ${\left[\mathrm{Co}(\mathrm{ox}{)}_3\right]}^{3-}>{\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}>{\left[\mathrm{Co}(\mathrm{ONO}{)}_6\right]}^{3-}>{\left[{\mathrm{CoF}}_6\right]}^{3-}$
For a given metal cation and fixed co-ordination number, 5 or 6 membered chelated complex is found to be more stable than complex with monodentate ligand.