I will stir things up a bit here and vote for Connie.

Here's why.

The family was always a patriarchal organization in which women played virtually no role whatsoever. The character of Connie over the course of the three films goes from the girlish bride swooning over Johnny Fontaine, to the abused spouse "spoiled guinea brat," to the person indirectly responsible for setting up Sonny, to an irresponsible mother and slut, and then grows to become arguably Michael's closest confidante and aide. It is she who orders the hit on Zasa, and it is she who poisons Altobello. She is the first Corleone woman to partake in hits. She is also the keeper of the dark secret of Fredo's murder by Michael as we see in the scene in GFIII when Michael tells her he went to confession.

It is also she who ushers Vincent into the family business and ensures his place as Michael's successor.

For sheer growth and development as a character, I give her the nod.