Some of my favorites: as to their significance...I can only say that they
are favorites....
Ginastera: Bomarzo
Poulenc: Dialogues and the Human Voice
Berg: Wozzeck
Barber: Hand of Bridge; Antony and Cleo; Vanessa
Janacek: House of the Dead
Hindemith: Mathis der Maler
Bernstein: Candide (yes, I see it as opera)
Tippett: Midsummer Marriage
Britten: Peter Grimes
Vaughan Williams: Sir John in Love
Penderecki: The Devils
Casella: La Donna Serpente
Korngold: The Dead City
Martinu: Julietta
Prokofieff: The Gambler; Fiery Angel
Rachmaninoff: Francesca da Rimini
Hanson: Merry Mount
etc.
Karl (who generally doesn't have much interest in opera)