Top ten American composers/songwriters

And coming from Schubert completely makes sense. It was between Barber and Copland. Copland more American imho.

Was Jim Morrison not mentioned?

Also, was the OP speaking of all genres or just rock/pop? If all, then maybe top 10 for each of the major genres: jazz, blues, pop, rock, classical...and each of these have distinctive sub groups. 

All genres. I would consider Morrison more on the performer side ie. Elvis, Sinatra, Armstrong. 

 Barber is Universal !
Nobody would think  Appalachian Spring  and  much of his work as  American without the titles . Copland  never did .


Barbers Ädagio is the unofficial National Anthem  in times of grief ..Should be playing right now .

To say Copland's music doesn't capture the American spirit...he incorporates our folk music, energy level, optimism.  And nothing against Barber. Just very hard to make choices. Some day someone will. The big question will be them referencing this thread. Lol.