Here is an excerpt from John XXIII's encyclical on the prodigious man honored today:
42. St. Leo, therefore, maintained that the Bishop of Rome, as Peter's successor and Christ's Vicar on earth, is the focal center of the entire visible unity of the Catholic Church. And St. Leo's opinion is clearly supported by the evidence of the Gospels and by ancient Catholic tradition, as these words show: "Out of the whole world one man is chosen, Peter. He is set before all the elect of every nation, before all the apostles and all the Fathers of the Church; so that although there are among God's people many priests and many pastors, Peter governs by personal commission all whom Christ rules by His supreme authority. Great and wonderful, beloved, is the share in its own power which the Divine Condescension assigned to this man. And if it desired other princes to share anything in common with him, never except through him did it accord what it did not deny to others."37
43. And since St. Leo regarded this indissoluble bond between Peter's divinely-given authority and that of the other apostles as fundamental to Catholic unity, he was never tired of insisting that "this authority to bind and to loose was also passed on to the other apostles, and what was established by this decree found its way to all the princes of the Church. But there was good reason for committing what was intended for all to the care of one in particular. And so it was entrusted to Peter individually because the figure of Peter was to be put ahead of all those in charge of the Church."38
44. There is, moreover, another essential safeguard of the Church's visible unity which did not escape that notice of this saintly Pope: that supreme authority to teach infallibly, which Christ gave personally to Peter, the prince of the apostles, and to his successors. Leo's words are quite unequivocal: "The Lord takes special care of Peter; He prays especially for Peter's faith, for the state of the rest will be more secure if the mind of their chief be not overthrown. Hence the strength of all the rest is made stronger in Peter, and the assistance of divine grace is so ordained that the stability which through Christ is given to Peter, should through Peter be transmitted to the other apostles."3945. Applied to St. Peter this pronouncement is clear and emphatic enough; yet unhesitatingly St. Leo claims the same prerogative for himself. Not that he wanted worldly honor, but he had no doubt whatever that he was just as much Christ's vicar as was the Prince of the Apostles.
You can read excerpts from St. Leo's sermons as well as Pope Benedict's angelus message about St. Leo at Insight Scoop.