There's no single one (although Merton isn't in the tops anyway).
In philosophy, City of God, Summa Contra Gentiles, Essay on Development of Christian Doctrine and Whose Justice Which Rationality are my favorites.
For literature, Gene Wolfe, Graham Greene, Sigrid Undset, Flannery O'Connor, Dante, Tolkien and Shusaku Endo are recommended.
What's the best Catholic book?
Caleb Murphy
Brayden Cox
On the Road, not memeing btw
Noah Martin
Swiss Family Robinson
Elijah Jackson
...
Colton Gray
La noche oscura del alma