Now I venture a theory for discussion, and please shoot me down if you think it is all wrong.
Modern music starts from about 1500. In the period 1500 till about 1650, primacy belongs to Italian, and to a lesser extent, English, composers. Palestrina and Monteverdi, obviously, but a big clutch of English (albeit some of them Catholic) men such as Tallis, Byrd, Tye, Orlando Gibbons.
Italy preserves for a while considerable talent (the Scarlattis, Vivaldi &c.), but the centre of gravity shifts to the German lands. Witness the Bach dynasty, the great age of Haydn and Mozart, Schubert and Beethoven and Mendelssohn and Schumann and Brahms and (dare we mention) Wagner, and we are not yet finished (Bruckner...)
The 20th century is that of American music in all its fascinating variety and inventiveness.
(a) the slave-plantation legacy of the negro spiritual, still enthusiastically sung in worship in all sorts of places
(b) the entire genre of jazz, in all its forms, improvised and written, for just about any ensemble combination
(c) close-harmony barber-shop
(d) country-and-western and blue-grass (someone please explain the difference?)
(e) the rich repertoire of film-music (Hollywood-inspired)
(f) several important classically trained composers, important in their own right (Gershwin, Copland, Bernstein)
(g) and I almost forgot, the thrilling Glenn Miller-like Big Band sound.
I exclude here only "rap", because that is just not music.
What think ye all?