The mention of Ives by Learsfool really got me thinking about who is the best expositor of America in his music and in what work does he express the American reality best.I suppose so many think of Ives in this respect because so many of his compositions were explicitly "Americana", like:
From the Steeples and the Mountains
Holidays Symphony
Thanksgiving
The Fourth of July
Three Places in New England
Variations on "America"
But, the composer who has always captured the American reality best for me is Aaron Copland, with his compositions like these:
Appalachian Spring
Billy the Kid - Ballet Suite
Fanfare for the Common Man
Lincoln Portrait
Quiet City
Rodeo
Symphony No. 3
The Red Pony
.