Most Influential American Musician of All Time?

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?

The original punk, Hank Williams Sr: Writer, performer, drunk.

How many musicians can honestly claim to ever approach Zappa's ability? By the same token the ridiculous guitar solos of the 80's "wee wee, weeeeee" went by the wayside and have been replaced with thoughtful and capable efforts by many fine guitarists. Say what you will but the two best guitar solos in the last 25 years that were considered to be commercial successes were Marillion's "Heart of Loriam" on the Misplaced Childhood album, and Pink Floyd's "Comfortably Numb" from The Wall. I would argue both of these pieces were infleunced by Frank Zappa. I can point to specific tracks if need be, suffice to say Zappa was one of the most innovative musicians ever. For your own good, listen to "Yo Mama" on the Sheik Yerbouti album, absolutely unbelievable. How about Black Napkins, Inca Road, or The Torture Never Stops? With all due respect Zappa paved the way and set an incredibly high standard for all to aspire to. Thankfully many have tried...

I would agree with Pawlowski that, aside from maybe Danny Elfman, nobody was influenced by Zappa (which does NOT diminish his greatness).

Far more people have been influenced by John Fogerty, who ranks high on my list.

I would nominate 1) Dylan, 2) Ellington, 3) Copland

Cheers

Dunno about all time...but how many rock guitarists (conciously or not) haven't absorbed Jimi Hendrix.

Thelonious Monk may not be a household commodity but he added alot of vocabulary and his influence on some of todays great players runs deep.