The first Leo was Leo the First (Duh) and is called Leo the Great. Among other things he convinced Attila not to sack Rome. He is also a Doctor of the Church. Leo 13th had a long reign and wrote Rerum Novarum, the first modern encyclical on social welfare and social justice, in 1891. He defended the rights of workers to fair wages and the right to unionize, while condemning Marxism and Socialism.  Pope Saint John Paul 2 wrote the encyclical Centesimus Annus on the 100th anniversary of Leo’s important social encyclical.

Great choice of name and I think it says much about the direction his Papacy will take.