For Ravel, the strongest contenders are
Daphnis et Chloe and
Gaspard de la nuit. They represent the pinnacles of his orchestral and pianistic outputs, respectively.
From the Medtner I have listened to, I might have to agree with you clavi.
Night Wind is by far the most complex and ambitious of his sonatas. Though if one wanted to pick a more mature work, the 2nd and 3rd piano concertos and the op. 53 sonatas (
Romantica and
Minacciosa) are strong contenders. Personally, I might pick the posth.
Piano Quintet, finished two years before his death and not published until afterwards.
Dichterliebe,
Fantasy in C, etc. are masterpieces of the highest order, but for me, there really isn't a dispute about what Schumann's magnum opus is. It is a mid-career work, the
Piano Concerto in a, op. 54. In the "pick your top 5 schumann" thread, this work appeared on every single list and at the top of exactly half of the lists. It's the closest Schumann ever got to composing a "warhorse". On the other hand, Schumann himself might have picked the extremely obscure choral work
Das Paradies und die Peri, which I myself am completely enamored with (listen to that ending!). But the strongest contenders besides the three mentioned above are probably
Frauenliebe und -leben, the op. 39
Liederkreis,
Kreisleriana,
Davidsbundlertanze,
Carnaval, and the groundbreaking quintet and quartet for piano.
Some composers intentionally worked towards a "magnum opus". Off the top of my head: Stockhausen's
Licht, Ligeti's
Etudes, Wagner's
Ring, Berlioz's
Les Troyens, and Prokofiev's
War and Peace (though I would disagree that this is his greatest work). Others composed works that in some way or another defined their entire career - for example Stravinsky's
Rite, Liszt's
Sonata in B minor, Messiaen's
Quartet for the End of Time, Janacek's
Mass, Pachelbel's
Magnificat-Fugues, Walton's
Belshazzar's Feast, Alkan's minor key etudes, and Verdi's
Otello.
Some other choices, conventional or unconventional...
For Brahms, I yield to Polednice.
For Handel, I would not pick the
Messiah, but rather the opera
Giulio Cesare.
For Bach, there are four: the
Art of the Fugue,
Mass in B minor,
St. Matthew Passion, and the
Well-Tempered Clavier.
For Beethoven, there is a similar situation: the late piano sonatas,
Missa Solemnis, 9th symphony, and 14th string quartet all have a good case going for them.
For Mozart, I may pick
Le Nozze di Figaro over
Don Giovanni, though both are strong contenders. He wrote a million other good works too - so take your pick:
Requiem,
Mass in C minor, late piano concerti, etc. I still think it should be an opera though, because that was the realm in which he dominated the most (well, besides Wagner).
For Prokofiev, I would disagree with the composer's pick of
War and Peace and choose instead the three War Sonatas. Piano Concerto #2 and #3 are perhaps his best works, but they are a little too early in his career to be "magnum opuses". Also,
Romeo and Juliet makes a good case for itself.
For Schubert, I agree with the three that Art Rock named.
For Richard Strauss, any of the three greatest operas would work. I would probably pick
Der Rosenkavalier because of its popularity. However, the
Four Last Songs might be even better picks, due to their sheer beauty and the place they held in Strauss's life. One can't ignore his orchestral works, of course, and gigantic masterpieces like
Eine Alpensinfonie,
Ein Heldenleben, and
Tod und Verklarung certainly make good cases for themselves.
Of Bartok's many masterpieces, I might pick the
Music for Strings, Percussion, and Celesta.
Bruckner's and Mahler's 9th symphonies are in many ways, culminating works of their entire symphonic careers.
Chopin's
Ballades make a good case for themselves, though as a set, his op. 28
Preludes, may have to grab the spot.
Villa-Lobos's
Choros and
Bachianas Brasileiras are logical choices, but the oddball piano masterpiece
Rudepoema seems to me equally ambitious and groundbreaking.
For Domenico Scarlatti, pick one of the 555 sonatas at random. It's too hard to choose. But if forced, I might pick the e minor sonata, K. 402, which Weston introduced me to.
