seriously? Gotta go with Willie on this one. The AMERICAN songwriter for the ages. (and I agree to an earlier post, he's kinda hard on the ears

)
Straight from Wikipedia:
Birth name Willie Hugh Nelson
Born April 30, 1933 (1933-04-30) (age 75)
Origin Abbott, Texas, U.S.
Genre(s) Country, Country Rock, Outlaw Country,
Alternative Country, Folk, Reggae, BluesOccupation(s) Singer, Songwriter, Guitarist, Producer, Session Musician, Actor, Activist
Instrument(s) Acoustic Guitar
Years active 1956 – Present
Label(s) Liberty, RCA, Atlantic, Columbia, Island, Justice Records, Lost Highway
Associated acts
Waylon Jennings
The Highwaymen
Johnny Cash
Kris Kristofferson
Ray Price
Merle Haggard
Johnny Bush
Julio Iglesias
U2
Kurt Nilsen
Thing is, I prefer other artists in various genre's, but if you want to say one guy is it, the american songwriter, the buck stops at Willie's place.
I'd give an honorable mention to Carole King also. So many songs that were hers, especially from "Tapestry", that most people associate with other artists on a nearly iconic level.