Fyodor Dostoevsky's The Idiot, Demons, and The Brothers Karamazov.
George Eliot's Silas Marner. (“George” was a woman, for those of you who keep track of these sorts of things.)
Frank Herbert's Dune.
Aldous Huxley's Brave New World. I thought Huxley hit the nail on the head, but after discovering a different sort of drug than the kinds his novel railed against, he seems to have expressed second thoughts in Brave New World Revisited.
Alessandro Manzoni's I promessi sposi (The Betrothed), if only for its superb recounting of the Plague of Milan.
Walter M. Miller, Jr.'s A Canticle for Leibowitz. I think Miller, like Huxley, hits the nail on the head, but he, too, had second thoughts later in life.
Luigi Pirandello's Il fu Mattia Pascal (The Late Mattia Pascal).
J.R.R. Tolkein's Lord of the Rings.
I don't really know enough about poetry for anyone to take this too seriously, thought I do think these poets insufficiently known.
Dante, of course. Some of his passages in are spectacularly evocative in Italian.
Era già l'ora che volge il disio
ai navicanti e 'ntenerisce il core
lo dì c'han detto ai dolci amici addio;
e che lo novo peregrin d'amore
punge, se ode squilla di lontano
che paia il giorno pianger che si more;… etc.
Gerard Manley Hopkins.
THOU art indeed just, Lord, if I contend
With thee; but, sir, so what I plead is just.
Why do sinners' ways prosper? and why must
Disappointment all I endeavour end?
Wert thou my enemy, O thou my friend,
How wouldst thou worse, I wonder, than thou dost
Defeat, thwart me? ...
Tallis' Spem in Alium was a magnificent surprise. Palestrina, Bach, Beethoven, … but some of my favorites are the ancient chants used at prayer, and the modern “folk” hymns used in Italian masses.