I collected over 18 lists of the best operas of all time from the Internet and books and combined them into one list.  The results are below, arranged by composer in order of the composer’s date of birth.  This list includes every opera on at least two of the original source lists (the number of lists is also included). For ‘best operas’ lists with even more information, go HERE for a list arranged by ranking (with the most listed operas at the top) or HERE for a list of the operas arranged by the date of their premieres.  

CLAUDIO MONTEVERDI (Italy, 1567-1643)
L’Orfeo (Orpheus) (1607) (11 lists)
L’incoronazione di Poppea (The Coronation of Poppaea) (1642-1643) (9 lists)

HENRY PURCELL (England, 1659-1695)
Dido and Aeneas (1688) (10 lists)

GEORGE FRIDERIC HANDEL (Germany/England, 1685-1759)
Rinaldo (1711) (2 lists)
Acis and Galatea (1718) (2 lists)
Giulio Cesare in Egitto (Julius Caesar in Egypt) (1724) (6 lists)
Rodelinda (1725) (2 lists)
Orlando (1733) (4 lists)
Ariodante (1735) (2 lists)
Alcina (1735) (5 lists)
Serse (Xerxes) (1738) (3 lists)
Semele (1744) (2 lists)

JEAN-PHILIPPE RAMEAU (France, 1683-1764)
Hippolyte et Aricie (1733) (3 lists)
Les Indes Galantes (The Gallant Indies) (1735) (5 lists)
Castor et Pollux (1737) (3 lists)

GIOVANNI BATTISTA PERGOLESI (Italy, 1710-1736)
La Serva Padrona (The Servant Turned Mistress) (1733) (6 lists)

CHRISTOPH WILLIBALD GLUCK (Germany, 1714-1787)
Orfeo ed Euridice (Orpheus and Eurydice) (1762) (10 lists)
Alceste (1767) (3 lists)
Iphigénie en Tauride (Iphigenia in Tauris) (1779) (5 lists)

NICCOLÒ PICCINNI (Italy, 1728-1800)
La Buona Figliuola (The Good-Natured Girl) (1760) (2 lists)

GIOVANNI PAISIELLO (Italy, 1740-1816)
Il Barbiere di Siviglia (The Barber of Seville) (1782) (4 lists)

DOMENICO CIMAROSA (Italy, 1749-1801)
Il Matrimonio Segreto (The Secret Marriage) (1792) (3 lists)

WOLFGANG AMADEUS MOZART (Austria, 1756-1791)
Idomeneo (1781) (7 lists)
Die Entführung aus dem Serail (The Abduction from the Seraglio) (1782) (7 lists)
Le Nozze di Figaro (The Marriage of Figaro) (1786) (18 lists)
Don Giovanni (Don Juan) (1787) (16 lists)
Così Fan Tutte (Thus Do They All) (1790) (10 lists)
La Clemenza di Tito (Titus’ Clemency) (1791) (2 lists)
Die Zauberflöte (The Magic Flute) (1791) (17 lists)

LUIGI CHERUBINI (Italy, 1760-1842)
Médée (Medea) (1797) (3 lists)

LUDWIG VAN BEETHOVEN (Germany, 1770-1827)
Fidelio (1805) (11 lists)

DANIEL AUBER (France, 1782-1871)
Fra Diavolo (Brother Devil) (1830) (3 lists)

CARL MARIA VON WEBER (Germany, 1786-1826)
Der Freischütz  (The Marksman) (1821) (5 lists)

GIACOMO MEYERBEER (Germany, 1791-1864)
Les Huguenots (1836) (7 lists)
Le Prophète (The Prophet) (1849) (2 lists)
L’Africaine (The African Woman) (1865) (2 lists)

GIOACHINO ROSSINI (Italy, 1792-1868)
L’italiana in Algeri (The Italian Girl in Algiers) (1813) (2 lists)
Il Barbiere di Siviglia (The Barber of Seville) (1816) (18 lists)
La Cenerentola (Cinderella) (1817) (6 lists)
Semiramide (1823) (4 lists)
Le Comte Ory (The Count of Ory) (1828) (2 lists)
Guillaume Tell (William Tell) (1829) (5 lists)

HEINRICH MARSCHNER (Germany, 1795-1861)
Hans Heiling (1833) (2 lists)

GAETANO DONIZETTI (Italy, 1797-1848)
L’Elisir d’Amore (The Elixir of Love) (1832) (8 lists)
Lucia di Lammermoor (1835) (12 lists)
La Fille du Régiment (The Daughter of the Regiment) (1840) (3 lists)
Don Pasquale (1843) (6 lists)

VINCENZO BELLINI (Italy, 1801-1835)
La Sonnambula (The Sleepwalker) (1831) (3 lists)
Norma (1831) (11 lists)
I Puritani (The Puritans) (1835) (4 lists)

HECTOR BERLIOZ (France, 1803-1869)
Benvenuto Cellini (1838) (3 lists)
La Damnation de Faust (The Damnation of Faust) (1846) (5 lists)
Béatrice et Bénédict (1862) (2 lists)
Les Troyens (The Trojans) (1863; 1890) (9 lists)

MIKHAIL GLINKA (Russia, 1804-1857)
A Life for the Tsar (1836) (2 lists)
Ruslan and Lyudmila (1842) (5 lists)

OTTO NICOLAI (Germany, 1810-1849)
Die Lustigen Weiber von Windsor (The Merry Wives of Windsor) (1849) (2 lists)

FRIEDRICH VON FLOTOW (Germany, 1812-1883)
Martha (1844) (2 lists)

RICHARD WAGNER (Germany, 1813-1883)
Die Fliegende Holländer (The Flying Dutchman) (1843) (9 lists)
Tannhäuser (1845) (11 lists)
Lohengrin (1850) (11 lists)
Tristan und Isolde (1865) (13 lists)
Die Meistersinger von Nürnberg (The Master-Singers of Nuremberg) (1868) (15 lists)
Der Ring des Nibelungen  (The Ring of the Nibelungen) (16 lists):
1. Das Rheingold (The Rhine Gold) (1869)
2. Die Walküre (The Valkyrie) (1870)
3. Siegfried (1876)
4. Götterdämmerung (Twilight of the Gods) (1876)
Parsifal (1882) (12 lists)

GIUSEPPE VERDI (Italy, 1813-1901)
Nabucco (Nebuchadnezzar) (1842) (5 lists)
Ernani (Hernani) (1844) (3 lists)
Macbeth (1847) (6 lists)
Rigoletto (1851) (15 lists)
Il Trovatore (The Troubador) (1853) (12 lists)
La Traviata (The Fallen Woman) (1853) (17 lists)
Les Vêpres Siciliennes (The Sicilian Vespers) (1855) (2 lists)
Simon Boccanegra (1857) (7 lists)
Un Ballo in Maschera (The Masked Ball) (1859) (4 lists)
La Forza del Destino (The Power of Fate) (1862) (6 lists)
Don Carlos (1867) (12 lists)
Aida (1871) (13 lists)
Otello (Othello) (1887) (12 lists)
Falstaff (1893) (15 lists)

CHARLES GOUNOD (France, 1818-1893)
Faust (1859) (9 lists)
Roméo et Juliette (1867) (12 lists)

JACQUES OFFENBACH (Germany/France, 1819-1880)
Orphee aux Enfers (Orpheus in the Underworld) (1858) (4 lists)
La Belle Hélène (Fair Helen) (1864) (2 lists)
Les Contes d’Hoffmann (The Tales of Hoffmann) (1881) (6 lists)

BEDŘICH SMETANA (Czech State, 1824-1884)
Prodaná Nevesta (The Bartered Bride) (1866) (6 lists)

JOHANN STRAUSS II (Austria, 1825-1899)
Die Fledermaus (The Bat) (1874) (5 lists)

ALEXANDER BORODIN (Russia, 1833-1887)
Prince Igor (1890) (5 lists)

AMILCARE PONCHIELLI (Italy, 1834-1886)
La Gioconda (1876) (4 lists)

CAMILLE SAINT-SAËNS (France, 1835-1921)
Samson et Dalila (Samson and Delilah) (1877) (5 lists)

LÉO DELIBES (France, 1836-1891)
Lakmé (1883) (2 lists)

GEORGES BIZET (France, 1838-1875)
Les Pêcheurs de Perles (The Pearl Fishers) (1863) (3 lists)
Carmen (1875) (19 lists)

MODEST MUSSORGSKY (Russia, 1839-1881)
Boris Godunov (1874) (13 lists)
Khovanshchina (The Khovansky Affair) (1886) (5 lists)

PYOTR ILYICH TCHAIKOVSKY (Russia, 1840-1893)
Eugene Onegin (1879) (14 lists)
Pique Dame (The Queen of Spades) (1890) (6 lists)

ANTONÍN DVOŘÁK (Czech State, 1841-1904)
Rusalka (1901) (4 lists)

JULES MASSENET (France, 1842-1912)
Manon (1884) (8 lists)
Werther (1892) (5 lists)

ARRIGO BOITO (Italy, 1842-1918)
Mefistofele (Mephistopheles) (1868) (4 lists)

NIKOLAI RIMSKY-KORSAKOV (Russia, 1844-1908)
The Legend of the Invisible City of Kitezh and the Maiden Fevroniya (1907) (3 lists)
Le Coq d’Or (The Golden Cockerel) (1909) (3 lists)

ENGELBERT HUMPERDINCK (Germany, 1854-1921)
Hänsel und Gretel (Hansel and Gretel) (1893) (3 lists)

LEOŠ JANÁČEK (Czech State, 1854-1928)
Jenůfa (1904) (10 lists)
Kát’a Kabanová (1921) (6 lists)
The Cunning Little Vixen (1924) (6 lists)
The Makropulos Affair (1926) (4 lists)

RUGGIERO LEONCAVALLO (Italy, 1857-1919)
I Pagliacci (The Clowns) (1892) (7 lists)

GIACOMO PUCCINI (Italy, 1858-1924)
Manon Lescaut (1893) (6 lists)
La Bohème (The Bohemian Life) (1896) (16 lists)
Tosca (1900) (15 lists)
Madama Butterfly (1904) (17 lists)
La Fanciulla del West (The Girl of the West) (1910) (4 lists)
Il Trittico: Suor Angelica (The Triptych: Sister Angelica) (1918) (2 lists)
Il Trittico: Gianni Schicchi (The Triptych: Gianni Schicchi) (1918) (3 lists)
Turandot (1926) (12 lists)

GUSTAVE CHARPENTIER (France, 1860-1956)
Louise (1900) (2 lists)

CLAUDE DEBUSSY (France, 1862-1918)
Pelléas et Mélisande (1902) (13 lists)

FREDERICK DELIUS (UK, 1862-1934)
A Village Romeo and Juliet (1907) (2 lists)

PIETRO MASCAGNI (Italy, 1863-1945)
Cavalleria Rusticana (Rustic Chivalry) (1890) (10 lists)

RICHARD STRAUSS (Germany, 1864-1949)
Salome (1905) (12 lists)
Elektra (1909) (10 lists)
Der Rosenkavalier (The Knight of the Rose) (1911) (15 lists)
Ariadne auf Naxos (Ariadne on Naxos) (1912) (5 lists)
Die Frau Ohne Schatten (The Woman Without A Shadow) (1919) (5 lists)
Capriccio (1942) (2 lists)

FERRUCCIO BUSONI (Italy, 1866-1924)
Doktor Faust (Doctor Faust) (1925) (4 lists)

FRANCESCO CILEA (Italy, 1866-1950)
Adriana Lecouvreur (1902) (3 lists)

UMBERTO GIORDANO (Italy, 1867-1948)
Andrea Chénier (1896) (5 lists)

HANS PFITZNER (Germany, 1869-1949)
Palestrina (1917) (3 lists)

FRANZ LEHÁR (Austria-Hungary, 1870-1948)
Die Lustige Witwe (The Merry Widow) (1905) (7 lists)

ARNOLD SCHOENBERG (Austria, 1874-1951)
Moses und Aron (Moses and Aaron) (1930-1932) (5 lists)

MAURICE RAVEL (France, 1875-1937)
L’heure Espagnole (The Spanish Hour) (1911) (2 lists)
L’enfant et les Sortilèges (The Child and the Spells) (1925) (5 lists)

BÉLA BARTÓK (Hungary, 1881-1945)
Bluebeard’s Castle (1918) (7 lists)

KAROL SZYMANOWSKI (Poland, 1882-1937)
King Roger (1926) (2 lists)

ZOLTÁN KODÁLY (Hungary, 1882-1967)
Háry János (1926) (3 lists)

IGOR STRAVINSKY (Russia/France/US, 1882-1971)
The Rake’s Progress (1951) (7 lists)

ALBAN BERG (Austria, 1885-1935)
Wozzeck (1925) (11 lists)
Lulu (1937) (5 lists)

SERGEI PROKOFIEV (Russia, 1891-1953)
The Love for Three Oranges (1921) (2 lists)
War and Peace (1946) (7 lists)
Betrothal in a Monastery (1946) (3 lists)

PAUL HINDEMITH (Germany, 1895-1963)
Mathis der Maler (Matthias The Painter) (1938) (4 lists)

VIRGIL THOMSON (US, 1896-1989)
Four Saints In Three Acts (1934) (4 lists)

ERICH WOLFGANG KORNGOLD (Austria-Hungary, 1897-1957)
Die tote Stadt (The Dead City) (1920) (2 lists)

GEORGE GERSHWIN (US, 1898-1937)
Porgy and Bess (1935) (12 lists)

FRANCIS POULENC (France, 1899-1963)
Dialogues des Carmélites (Dialogues of the Carmelites) (1957) (6 lists)

KURT WEILL (Germany, 1900-1950)
Die Dreigroschenoper (The Threepenny Opera) (1928) (7 lists)
Aufstieg und Fall Der Stadt Mahagonny (Rise and Fall of the City of Mahagonny) (1930) (4 lists)

MICHAEL TIPPETT (UK, 1905-1998)
The Midsummer Marriage (1955) (2 lists)
King Priam (1962) (3 lists)

DMITRI SHOSTAKOVICH (Russia, 1906-1975)
Lady Macbeth of the Mtsensk District (1934) (8 lists)

OLIVIER MESSIAEN (France, 1908-1992)
Saint François d’Assise (St. Francis of Assisi) (1983) (3 lists)

GIAN CARLO MENOTTI (Italy/US, 1911-2007)
The Consul (1950) (3 lists)
Amahl and the Night Visitors (1951) (2 lists)

BENJAMIN BRITTEN (UK, 1913-1976)
Peter Grimes (1945) (11 lists)
The Rape of Lucretia (1946) (3 lists)
Billy Budd (1951) (7 lists)
The Turn of the Screw (1954) (6 lists)
A Midsummer Night’s Dream (1960) (6 lists)
Death in Venice (1973) (3 lists)

BERND ALOIS ZIMMERMANN (Germany, 1918-1970)
Die Soldaten (The Soldiers) (1965) (2 lists)

GYÖRGY LIGETI (Romania/Hungary/Austria, 1923-2006)
Le Grand Macabre (The Grand Macabre) (1978) (5 lists)

HANS WERNER HENZE (Germany, 1926-2012)
Elegy for Young Lovers (1961) (2 lists)
The Bassarids (1966) (2 lists)

CARLISLE FLOYD (US, 1926- )
Susannah (1955) (2 lists)

PHILIP GLASS (US, 1937- )
Einstein on the Beach (1976) (4 lists)

JOHN ADAMS (US, 1947- )
Nixon in China (1987) (4 lists)

JUDITH WEIR (UK, 1954- )
A Night at the Chinese Opera (1987) (3 lists)