major mike - Elvis the first 'superstar"?
I think ol' blue eyes would probably have you pistol whipped by Sammy and Deano for making that claim.
BTW, there's three great American composers that certainly should be up for consideration: George Gershwin, Leonard Bernstein and Elmore Bernstein (no relation to Leonard.)
And certainly you shouldn't look past our own American "Royalty" the Duke of Ellington and the Count of Basie.
I'll be the first to admit, when you start looking at lists like this, the first instinct seems to be give more "weight" to that music and those performers that you are more familiar and those from earlier days don't get the consideration to which they are deserving.
So a guy like George M Cohan, who hit his peak circa WWI just isn't going to get pushed to the front of the line, like perhaps a Michael Jackson will, even though arguably he had much more influence over shaping American music than Jackson.