Augustine was the greatest theologian. He is by the way very different from Luther and Calvin. He is a greater psychological genius than any other theologian and his insights on the relation between freedom and grace and the human condition before and after salvation is spot on. That is should be considered as just a fact.
You should add to your patristic reading St. Ignatius of Antioch, St. Irenaeus, St. Theophilus of Antioch, St. Hippolytus of Rome, St. Clement of Alexandria, Origen, St. Athanasius, the Macarian homilies, St. Ephraim the Syrian, the three Cappadocian fathers, St. John Chrysostom, St. Cyril of Jerusalem, and St. Cyril of Alexandria
Then, you can come back and tell us which is the greatest theological genius. Don't tell us who you think is the best theologian until you read all these men, who also received similar praise by many others as "greatest theologians".