So hard to decide, but here's my top 20 (in no particular order) (and not including opera composers like Puccini, Wagner, Verdi)
- Bach, Mozart, Beethoven, Schubert, Brahms
- Tchaikovsky, Schumann, Bartok, Mendelssohn, Dvorak
- Handel, Haydn, Chopin, Debussy, Ravel
- Stravinsky, Mahler, Shostakovich, Rachmaninoff, Prokofiev
And I still missed a lot of greats!