The $E^{0}$ value for $M^{3+}/M^{2+}$ measures the stability of the $+2$ state relative to the $+3$ state. Cobalt ($Co$) has a very high positive value ($+1.97\text{ V}$) because the $Co^{2+}$ ion is significantly more stable than the $Co^{3+}$ ion in simple compounds, making the reduction very favorable.