“Sousa and Scott Joplin are probably the two most enduring American composers of their time.”
Add Stephen Foster to that and I can agree. I think he was probably the most prolific and many of his >200 songs are recognized without anyone realizing their origin.
(Old Kentucky Home, Camptown Races, Old Folks at Home, Oh Susanna, etc etc)
I'd say Irving Berlin, though many might not know the name, and certainly Francis Scott Key.
Regarding Stephen Foster, please see my old thread Who is Stephen Foster? (Now at 235 comments)
ML/NJ