Most Influential American Musician of All Time?

Elvis was a very good singer, a poor to fair guitarist, barely a songwriter, and a mediocre (benefit of the doubt on this one) actor.

He was a great entertainer but hardly a musician.

I agree that the question of who is the most influential is subjective and, therefore, has no definitive answer. But I think this question is far less subjective than "who is the greatest."

Although I had initially suggested Louis Armstrong, and still feel that he is right up there, subsequent posts have me thinking more about Robert Johnson and Bob Dylan. All three of these musicians seem to have influence that goes way beyond their own musical genres. Can the same be said of, for example, Gershwin, Berlin, or Grandmaster Flash?

Frank Zappa, Leonard Bernstein, Robert Zimmerman, Miles Davis and Frank Sinatra

Can I have an example of someone (who in their own right have made a significant contribution to American Music) who's been influenced by Frank Zappa?

Gershwin is probably my favorite American songwriter/composer on any given day but the question is influential musician not greatest songwriter. Is Gershwin even the most influential songwriter of the jazz era? I don't think he is. That would probably go to WC Handy or maybe Scott Joplin. Their music is at the root of blues and jazz respectively.

Some of the answers are getting off point IMO and yes, this is purely a subjective exercise, none of it can be proven. A great case can be made for Elvis. How many ten's of thousands of impersonators has his influence begat? All seriousness aside he is a major influence as a musician? Yes, if you count a singer in the equation but then the question should be most influential singer? While a singer is technically a musician it never entered my mind to consider individuals know as singers (i.e. Sinatra, Presley, Fitzgerald, Vaughn, Holiday, Madonna etc) or songwriters (i.e. Gershwin, Porter, Arlen, Berlin, Dylan). I certainly consider many of the above excellent musicians but their influence is greater as
Singers and/or Songwriters. I was thinking along the lines of a musical instrument musician. Is that the intent of the question or do singers and songwriters count as well?

Leonard Berstein and Arthur Fiedler are probably the most influential personalities that brought orchestral and classical music to the masses. Does anyone remember Berstein's show in the 50's something like "Young people's guide to the Orchestra" I was greatly influenced by that show as a kid as I'm sure many more were. But for the question posed I don't think these guys count. The question requires narrower boundaries. What do you say Jeffery, its your thread?