Most Influential American Musician of All Time?

Who do you regard as the most influential (i.e., musically, not necessarily commercially) American musician of all time across all genres?

The more I learn about Louis Armstrong, the more I am persuaded that he deserves that honor.
T-Bone Walker was more influential than Muddy Waters. His writing, singing, guitar playing and dancing were a major influence on Muddy, B.B. King and Chuck Berry. Jimi Hendrix wasn't the first to play the guitar behind his head.

That said, my vote goes to Mr. Louis Armstrong. He invented modern popular singing. 2nd place goes to Nat King Cole. Every piano lounge player is channeling Nat. 3rd place - tie between Elvis and Frank S.
I have to agree with my colleagues:

Gershwin and Ellington without a doubt. Their music extends way past the time and idiom in which it was written. And I think that many on the thread confuse great performers and performances with great music composition and its influence, especially on other musicians.

I don't want to exclude anyone but without those two creators the canon of American music gets pretty thin, pretty fast.

On a more humorous note, i am reminded of the SNL skit from 1975 when Chevy Chase announces that the first message from intelligent life from outer space has been received in response to our Pioneer One spacecraft that had our location, periodic table, prime numbers, and selections of music by Mozart, Chuck Berry, and poetry by Robert Frost embedded in a gold recorded disc.

"And the message is a simple four word message: 'Send More Chuck Berry'"

a classic.

Nina Simone
Louis Armstrong hands down. He and his peers created solo improvisation as we know it today and his vocal phrasing forever changed the way singers sing. There isn't ANYONE in popular western music outside of classical music that does not owe much of what they do and how they do it to Louis "pops" Armstrong. Robert Johnson was important to the development of country blues and later ,by extension, R&B and R&R as well as hard bop. However everything in modern R&B,R&R as well as people like Sinatra,Bennett,Vaughn,Holiday,Fitzgerald,Nat Cole,Miles,Coletrane,Simone owe much of what they do to Pops. Duke Ellington was and is an enormous force in western music. He wanted to extend the musical field beyond boundries like "jazz" and "classical". He played dance music but he always wanted to write (and did) truly American popular long format "classical" music. I give Pops the nod because as a country (and as a world) we have gravitated more to rock, pop, jazz,small goup instumental or vocalist than to complex popular large group arrangements. ( yes I know Duke had plenty of small group jazz sessions but in them even he is indebted to Pops) This is all just pure opinion on my part. Feel free to fire away if you disagree. I love this stuff. - Jim