The first is easy for me, Pope John XXIII. He is probably the biggest reason I am a Catholic today. He put a human face on the Church, and made it seem relevant to what was happening in the world and with humanity. To me, he led Catholicism out of the dark, cold, somber shadows of a Latinized European religion that overly stressed sin, shame and punishment, and showed it to be a gospel oriented Church of faith, love and hope.
My number two right now is Pope Francis. His focus on a forgiving, merciful and welcoming church is also gospel oriented. I really liked both of his first major documents and in putting the spotlight on issues of the family and marriage (and re-marriage), and his focus on a year of mercy. I love his simpler, low key style, and how he does not put himself above the rest of us, but acts like the servant of all of us. And expects the same of his fellow bishops.
A somewhat distant third would be Pope Paul VI, primarily for how he opened the Church to the world, and by being the first to travel extensively, started to change the view of a church revolving around Rome and the Curia, to one of a Church centered on Christ and encompassing all the faithful equally.
Being most familiar with popes of the past 200 years, it would be difficult for me to rank other popes before that. From what I have read, however, there have been other excellent popes over the centuries.