Top ten American composers/songwriters

I think we've reached ten.

Gotta toss in Irving Berlin.

OMG. People: Neil Diamond.

Don't Happy Be Worry. - Eugene Chadbourne

Kevin Gilbert
Andrew Gold
Will Owsley
Mose Allison
Terry Adams
Jimmie Webb  - Great solo showman

Stephen Foster
Born 1826 Pennsylvania 

Hank Williams
Chuck Berry
Bob Dylan
John Fogerty
Paul Simon
Stevie Wonder
Carole King
Randy Newman
Bruce Springsteen
Tom Petty

That's my list for popular music.  For me, Randy Newman comes in just before James Taylor.  Joni Mitchell and Neil Young are ineligible, since they were both born in Canada.  I would have to make another list for jazz (Monk, Ellington, Shorter, etc.), and Broadway (Gershwin, Rogers and Hammerstein, Stephen Sondheim, etc.). 

Some good candidates already mentioned. Also:

Samuel Barber
Walter Piston
Howard Hanson
John Adams
Elliot Carter
Phillip Glass (reluctantly and depending on how one defines “top” 🤔)

In Jazz and popular music:

Billy Strayhorn
Wayne Shorter
Benny Golson
Thelonious Monk
Cole Porter
Ervin Berlin
Hoagy Carmichael
Jerome Kern
Stephen Sondheim
Oscar Hammerstein
Stevie Wonder

Robert Ellsworth Pollard at the top of the list. 

A lot of the greats already listed. I’ll add Brian Wilson, Chuck Berry, Dave Bartholomew, Pomus/Shuman, Leiber/Stoller, Willie Dixon, Buddy Holly, Hank Williams, Merle Haggard, Randy Newman, Jackson Browne, Iris Dement, John Hiatt, Rodney Crowell, Felice & Boudleaux Bryant, Goffin/King, Greenwich/Barry, Mann/Weil, and Holland/Dozier/Holland (writers of the sublime "What Becomes Of The Broken Hearted").

Foster, as in David Foster......he’s a Canadian also.  Perhaps the OP meant North American

Keep ’em coming. A lot of quality names. Leonard Cohen, however, is a Canadian. Ahem...  But Joni can be counted in because she abandoned Canada for New York and then L.A.

Tod Rundgren
Greg Allman
Paul Simon
Alan Hovhaness
Joni Mitchell
Ferde Grofé
J.P. Sousa
Woody Guthrie
Huddie Ledbetter
Louis Armstrong

Paul Simon
Tom Petty
James Taylor
Carol King
Barry Manolo
Bruce Springsteen
Bob Dylan
Leonard Cohen

Good list.  But Frank Zappa is certainly a contender.    So is Leonard Bernstein.  Laura Nyro.  Eric Whitacre.  Miles Davis.  Charles Ives.  So is Jimmy Webb.  Don't put down Jimmy!