As other have mentioned, Ven. Matt Talbot for alcoholism, St. Maximilian Kolbe for drug addiction, St. Augustine for loose living.
But the saints that popped into my head while reading your post were Blessed Charles de Faucauld and Blessed Pier Giorgio Frassati and to some extent St. Ignatius Loyola, whose feast day is today.
Blessed Charles de Faucauld led a very colorful early life, full of wine, women and other vices.
“For twelve years,” he would later write of this period of his life, “I denied nothing, but believed nothing. I lived as one can when the last spark of faith has gone out.”
One of his cousins was a very devout Catholic and she wrote him letters, gently poiting out a path of peace. Charles was also very affected by waiting Muslims pray devoutly five times a day, completely absorbed in their worship of God.
One night he went to a church and kneeling before an altar, he prayed to God, “My God, if you exist, make yourself known to me.”
God did make Himself known, and Charles reverted back to his childhood faith. He devoted himself to the hidden life of Jesus and intended to start a religious order to honor that aspect of Christ, which didn’t come into fruition until after his death.
Bl. Pier Frassati did not lead a young life of vices, but he is considered the patron of youth. Every time you see a picture of him, he is smoking cigars with friends, mountain climbing, or just hanging out. He knew how to be a devout teenager while still having a good time. I feel that he is very interested and eager to talk to young men who are struggling to lead a life of faith. He would be a great friend.
St. Ignatius Loyola was a soldier before his conversion and according to some gossip, fathered two children (though this might be total hogwash). His writings are terrifically helpful for a revert or convert to Catholicism, because he specifically outlines all the temptations the devil lays before us on our struggle to lead a devout life.