Gregory the Great
Gregory the Great (540 - 604) became Pope in 590, at the death of Pelagius I. He actually disclaimed the title Pope, but wielded all the power of the office, effectively asserting the dominance of the bishop of Rome over all others. He was the first pope to do this effectively, which is one reason that his reign is considered the first of the Middle Ages.
He came into conflict when the patriarch of Constantinople claimed the title "ecumenical bishop", which mirrored the power and position he had sought for the papacy.