Father Robert Barron states that Evelyn Waugh's Brideshead Revisited should receive that title. Allow me to parse the phrase: It is a novel; the author is Catholic; it was written in the 20th century; it is set in the 20th century; it is about a Catholic family and the protagonist's interactions with each member of that Catholic family; it is well-written, plotted, and filled with fascinating characters; it seems to fit the bill.
Charles Ryder meets Sebastian Flyte at Christ Church in Oxford and becomes his friend (and more?); Charles becomes the lover of Julia, the confidante of Cordelia, the antagonist of both Lady Marchmain and her eldest son, Bridey (the former because of his relationship with Sebastian, particularly his aiding and abetting Sebastian's alcoholism; the latter because of his relationship with Julia, particularly his leading her into "living in sin"), and finally the witness of Lord Marchmain's reconciliation to Jesus and His Church.
Through all the evocation of Oxford, the beauty of Brideshead, the curiosity of a Catholic noble family in Anglican England, all the tangle of relationships and conflicts, Waugh stated that real theme of the novel was the operation of God's grace in the Marchmain family and by extension, Charles, as he is a member of that family. Although none of the characters achieve a worldly happy ending, each of them receives grace:
Lady Marchmain remains true to her Catholic faith: she makes mistakes in handling the faults of others around her (her husband's infidelity, her son's drunkenness, etc), but she is faithful. She is not perfectly good; but no one in the novel is perfectly bad, either. She is like Bridey, staunch and strict: we don't really like her, but she has integrity.
Lord Marchmain returns home to Brideshead and home to the Church, and through his conversion, he brings Julia back to the Church: she feels the twitch upon the thread and knows that she cannot commit adultery with Charles. Whatever else Lady Marchmain else accomplished, Julia knows her Catholic faith, like Sebastian, she has the vocabulary of Catholicism, as when she acknowledges that she cannot choose a lesser love (Charles) instead of God!
Sebastian finds grace even in his drunkenness; Cordelia, who shares some of her mother and her brother's attributes, is my favorite character in the novel. She is the one who sees the dangers Charles and Julia face and also appreciates the holiness that Sebastian finds as the doorman at a monastery in Northern Africa. I always enjoy reading the scene in which Cordelia tells Charles the story of Sebastian's life at the monastery. She sees with eyes of faith the grace that Charles misses, even in her, when she accurately states his impression of her after returning from the Spanish Civil War: "thwarted"--because then she turns the term right back on him. She has prayed for his conversion and she works for her father's return to the Faith, because Cordelia believes in heaven and wants to go there after death: she is simple and profound.
Finally, Charles himself experiences conversion: he recognizes the grace that Lord Marchmain has received, and that it is "no little thing"; through that sign of Lord Marchmain on his deathbed, Charles realizes that there is a God--and he can even accept Julia's decision with a kind of bitter regret, yet understanding. The last scene, in the chapel Lord Marchmain built for his Catholic wife, demonstrates Charles' conversion and faith.
I do think Brideshead Revisited is the best Catholic novel of the twentieth century; it may be one of the best Christian novels on the twentieth century as it depicts the providence of God working in one family's, one man's life. Waugh's special insight is that he understands the human person in the fullest Catholic sense--fallen, redeemed, receiving God's grace and love all unmerited and yet participating in that grace with free will; he depicts the sacramentality of the Catholic Christian life. And although Waugh is dealing with very serious matters--life and death; love; eternal salvation or eternal damnation--there is a brilliant sense of humor and of the absurd (Rex Motram's attempt to become Catholic; Mr. Samgrass trying to keep up with Sebastian; Charles at home with his father during the summer break, etc).
If you haven't read it, I think you should; I reread it every other year. The 1981 Granada tv series is a good supplement; avoid the recent movie adaptation like the plague, or like Mr. Samgrass.