I am becoming increasingly convinced that besides the Bible and the Catechism of the Catholic Church, the works of these men are the most beneficial ever written. This is from a standpoint of revelatory knowledge, not necessarily devotional or reflective. For those I would add Sts. Francis of Assisi, Teresa of Ávila, and Thérèse of Lisieux.
St. Augustine of Hippo
St. Thomas Aquinas
St. Louis de Monfort
Blessed John Henry Newman
G. K. Chesterton
C. S. Lewis
Venerable Fulton J. Sheen
Pope Benedict XVI/Joseph Ratzinger (Future Doctor of the Church, I hope)
I am sure there are more, but I only have less than half a lifetime left. Just Chesterton alone would take the rest of my life to read, reread, and fully appreciate his work.
C. S. Lewis is the only non-Catholic in this list. Lewis was very Catholic-like, and it has been said that if he could have gotten past his mental block regarding the primacy of the Pope, he very likely would have become Catholic. But then, if he had become Catholic, he would likely not be as popular with Protestants as he is. A Protestant may not realize it, but when he is reading Lewis, he is actually reading Catholic theology.