That's put me on the spot choosing a favourite Graham Greene provencalroseparadox
I'm not religious at all but I really love his 'Catholic' novels, all the anguished attention to the details of faith, souls teetering on the brink of immortality, but for this or that transgression. The Heart of the Matter I think explored this really well, I remember its impact, the double bind of loving God and loving other human beings. The end is heartbreaking and skilfully done.
Having said all that, Brighton Rock and The End of the Affair I have re-read for pleasure, love the tension and build up to a brilliant conclusion in the former and, just, men and women, love and loss in the latter.
Our Man in Havana vv funny and the first one I read, so have a soft spot for it.
I kind of want to reread to see if my feelings will have changed.
Not answered this terribly well have I?! 
I have The Quiet American and Monsieur Quixote sitting on my tbr shelf, long overdue their turn, so may update soon.