I'm not sure there would ever be a clear cut answer to the question. As we have seen from the comments on this thread, several people have made contributions without which, it could be argued, rock and roll would not exist.
Thank you to Ben Franklin for figuring out how to harness lightning (electricity). Thank you to Thomas Edison for figuring out how to make it possible to bring light into homes. Thank you to Leo Fender, Les Paul, and countless other inventors for their contributions.
Musically, I think you go back to Robert Johnson. But even then, you have to look at his influences for helping shape his sound.
Muddy Waters sang a song once called "The Blues Had a Baby and Named it Rock and Roll." He had it right, but you also have to put gospel music in the DNA as well, since the Blues was influenced by gospel.
Having said all that, if I had to name one person who might be the most important person in the development of Rock and Roll, I would say (as I think someone else already has) Sam Phillips. He said if he could find a white boy who sounded black, he could make a lot of money. While Elvis didn't exactly sound black (to me, anyway) he certainly didn't sound like the polished white-bread popular singers of the day. Thus, white kids everywhere...here and abroad...were introduced to music they had never, ever heard before. And that, I believe, started it all.
I don't know if someone else would have discovered Elvis, but as is usually the case when momentous events happen, Sam Phillips was in the right place at the right time.
So I guess my vote would go to Brother Sam.
Mike