Massenet (Jules), who lived into the 20th century, wrote some of the great French Romantic ballets, oratorios, and especially operas, though their popularity has gone up and down over the past century. Nevertheless several of them remain in the standard repertoire, including Hérodiade, Manon, Esclarmonde, Werther, and Thaïs. Symphony lovers, meanwhile, can enjoy the heady sensualism of his seven orchestral suites, which are lighter than symphonies but shorter than operas!
Medtner (Nikolai) was a Russian piano virtuoso who wrote complex, difficult, and powerful music, chiefly for the piano. His three concertos, 14 sonatas, 38 "Fairy Tales," and 2 sets of "Forgotten Melodies" for that instrument constitute the main body of his work, though he also wrote many songs, three violin sonatas, and an important piano quintet (i.e., a piece for one piano, plus string quartet). Medtner himself recorded many of his works, and several great pianists have championed them, but they remain a rare taste treasured by a passionate few.
Méhul (Étienne), the leading composer of the French Revolution, was the first composer to be designated "Romantic." His notable operas include Euphrosine, Ariodant, and Joseph. He also wrote five symphonies, of which the 3rd and 4th were only recently discovered, and the 5th is unfinished.
Menotti (Gian Carlo) was a twice-Pulitzer-winning, Italian-American opera composer, known for works that blur the line between popular theatre and art music. Among his operas still regularly performed are The Consul and The Saint of Bleecker Street. Amahl and the Night Visitors was the first opera composed for American television; it premiered on the Hallmark Hall of Fame. It is a Christmas tradition, as The Medium is a Halloween one; and the one-act opera The Telephone was the first opera I ever listened to! Menotti also wrote symphonies, concertos for piano and violin, ballets, cantatas, a stage play, and the librettos for two of Barber's operas.
Merulo (Claudio, a.k.a. Claudio Merlotti) was a late-16th-century organist in Venice and Parma who published two books of toccatas in which contrapuntal sections alternate with "passagework" (i.e., "filler music" demonstrating the performer's virtuosic skill). His music is prophetic of Baroque styles of composition that developed after his death. Merulo also published books of madrigals, and was the dedicatee of a treatise on keyboard playing by Diruta.
Messiaen (Olivier) was one of the 20th-century's more colorful and independent composers. Too great, indeed, to be covered in just one paragraph! His Quartet for the End of Time was composed (and first performed) in a Nazi POW camp. He also wrote massive works for orchestra and for organ, drawing inspiration from birdsong, Catholic mysticism, and legends such as Tristan and Isolde. Among these works is the controversial Turangalîla-Symphonie, a tribute to sensual love; the opera Saint-François d'Assise; the organ collection Book of the Holy Sacrament, the piano pieces Visions of the Amen and Twenty Gazes on the Infant Jesus, the oratorio The Transfiguration, and a spectacular orchestral piece called The Ascension, which I once heard from onstage at the St. Louis Symphony (the chorus was singing in the same half of the program, and I was seated so close to the percussion that I could have reached out and touched the bass drum - which rocked my world).
I just have to share one other personal anecdote about Messiaen. When I was in college, my organ teacher assigned me one of his pieces. It was called The Celestial Banquet (or French equivalent) and was actually not terribly difficult. The trickiest part was the long, steady ritard; one had to count every single beat, and gradually slow down from beginning to end. There was also a passage in which the score instructed that a series of staccato pedal notes be played "like drops of water," and this, my teacher alleged, originally read "like drops of blood." Whatever it was supposed to sound like, it felt really good to play. I would come into the practice room stressed out, breathless, every nerve ending frayed, and by the time I had finished playing this short piece, I felt completely at peace. A massage could not have been more relaxing!
Meyerbeer (Giacomo) was a founding figure in "grand opera." He wrote big, spectacular, expensive-to-produce operas, and was the last composer to write for castrati. Though Meyerbeer's popularity has suffered in the shadow of Wagner (who viciously attacked him, mainly for personal and antisemitic reasons), some of his works are still performed, such as Robert le Diable, Les Huguenots, and L'Africaine.
Milhaud (Darius) is one of Les Six, that group of French composers I promised never to mention again, but I'm reneging on that promise in just this case and one other. Milhaud was a ridiculously prolific composer, who experimented with polytonality and the blending of jazz and fine-art music. His best-known works are the ballet La Création du Monde and the saxophone rhapsody Scaramouche. He also wrote symphonies (12), string quartets (18), operas, concertos, and an autobiography titled My Happy Life.
Monteverdi (Claudio) is the most notable composer representing the transition from Renaissance to Baroque music, a transition one can trace through his nine books of madrigals. Since the birth of the Baroque is tied to the birth of the opera, it should be no surprise that Monteverdi wrote the earliest opera still performed today: L'Orfeo. His other well-known works include The Return of Ulysses, The Coronation of Poppea, and the Vespers of 1610.
Mussorgsky (Modest) was a Russian composer of gigantic talent who focused his energies on writing nationalistic music, based themes from Russian folklore and history, and flavored by Russian folk music. Many of his best-known pieces, including Night on Bald Mountain, Pictures at an Exhibition, and the opera Boris Godunov, were either orchestrated, revised, or completed by other composers after his death. A member of "the Five," a circle of like-minded Russian composers who mostly made their living in other fields (Mussorgsky himself was an army officer and government official), he seems to have had most of the creative genius in the group, but it went to waste as he slowly drank himself to death. Other members of the Five include Borodin (whom I have already discussed), Balakirev and Cui (W.T.S. yourself), and Rimsky-Korsakov, whose name I dropped in my original list of "great composers," so that's that.
Myaskovsky (Nikolai), whom Wiki calls "the father of the Soviet symphony," wrote 27 symphonies and 13 string quartets, plus piano sonatas, tone poems, concertos, cantatas, etc. Heck, here is a complete list of his works. Obviously this guy has a serious following, though I have only heard one or two of his symphonies myself. One of my goals is to look into his work a good deal more, since Soviet music is one of my areas of interest. I'll let you know what I find!