Reluctantly I had to vote for Buddy Holly for the very reason Peter said he didn't include the Everly Brothers - more pop that rock'n'roll.
Rock'n'Roll, I'm afraid, was always a minority taste, except perhaps for a brief period in the late 1950s among teenagers.
We are talking about who was the most 'influential' here, and Buddy Holly was years ahead of his time, which is precisely why he is my least favorite of the 1950s Rock'n'Roll artists. I don't like the strings or some of the poppy arrangements of some of his songs. My favorite of his is probably 'Rockin' Around With Ollie Vee'.
Bill Haley certainly started the whole Rock thing, among white folks worldwide anyway. But he is hardly a big influence, he basically took black rhythm'n'blues, cleaned it up and recorded it for a white audience, and it took off.
I don't see that any of the original rock'n'roll artists mentioned had a huge influence, but Buddy's recordings sound more like the music of the past 40 years or so than any of the others. Chuck Berry influenced a lot of 1960s guitarists, but Jimmy Hendrix probably had more influence on how they've played guitars since.
I think nearly all the original rock'n'roll stars mentioned had their own unique sound, and nobody has been able to copy their style, or at least nobody has been so successful at it they became super-stars.
I suppose that's a compliment in itself. Nobody sings like Elvis, nobody really famous today plays piano anything like Jerry Lee, nobody sings like Little Richard, or Fats Domino, or Gene Vincent, etc.
But the softer sound of Buddy Holly, and the Everly Brothers come to that, is mostly what I've heard in popular music since the hard sounds of 1950s rock'n'roll left the charts around 1959.