The Holy Father has in the highest degree the roles of Priest, Prophet, and King. Of the many things he has done and said, three stand out in my mind as having a lasting influence on the Church.
As priest, he laid a solid foundation for the “Reform of the Reform” of the Roman liturgy with his moto proprio, Summorum Pontificum, of 07/07/07. Its deepest purpose is the “interior reconciliation” of the old Mass and the new. The old Mass is now universally available as a “standard of continuity” and so the two forms are in a position to be “mutually enriching.” Some, even in the hierarchy, are hoping eventually for a “Missal of Convergence.”
As prophet, he not only exposed a false prophet whose followers threaten to overwhelm Europe demographically, but he laid another solid foundation, the only one on which genuine dialogue with Muslims can be constructed: the affirmation of the validity of human reason. He did this in his remarkable address at the University of Regensburg on September 12, 2006. But his prophetic critique was not only, or even primarily, directed at Islam. He called the West back to a deeper understanding of reason that goes beyond the self-limitation of modern science to what can be counted and measured—to reason that is open to transcendence.
On a side note, as prophet—besides three foundational encyclicals, he has given the world an extended theological and spiritual biographies of the great figures of the Church from the Apostolic Era to (so far) the Middle Ages.
As king, he established the canonical basis for personal ordinariates that would permit entire Anglican parishes and dioceses to return to full communion with the Catholic Church. The apostolic constitution Anglicanorum Coetibus is arguably the most important event in Anglican-Catholic relations since 1534.