In my opinion there simply cant be a single greatest. There are many "greatest". The reason being is that Rock as we know it is in continuous evolution. Let me explain, form its roots of R&B every artist in one way or the other contributed to the development of rock and roll, be it with a particular playing tecnique, or with a way of moving on stage (which I would like to remind you that in the late 50's and 60's was considered obscene) to the way of singing etc... so we reach the turning point with Elvis, who was able to condense in his act the best of what had been developing until that time. The next big step is not The Beatles... its Dylan. Finally lyrics became a serious issue, and folk became Rock. He was the one who amped his guitar and blasted the volume, to the extent that in the concert where this took place he was booed... then came The Beatles, but had they continued to play love me do's and please please mes they would have not gone very fare had it not been for external inspiration. What I mean by this is, for example an encounter that they had with Dylan where he basically told Lennon, "hey your sound are cool, but your songs don't say anything... " so that the point where the beatles lyrics started changing... then came another external influnce... the sounds of another band.. which was not the Rolling Stones, but were the Beach Boys... there was a certain competition between them... musically speacking the Beach boys were better than the beatles to the point that their album "pet sounds" is one of Paul's favorite, so from there the Beatles started making more intricte music in in the span of just about six years went from three chorded songs to complex music with all sorts of instruments and arrangements.
Then came Hendrix who tought the world, and I mean everybody that was in the business how to play guitar. Hendrix became popular in the UK before really hitting it big in the States... everyone went to see him and everyone from the who, to clapton, to the beatles... were simply stunned at what this guy did with his guitar. With Hendrix a different type of stage presence came to be... an act that was then put together by the Rolling Stones, hard rock, flamboyant clothing, sexual ambiguity etc... Then came the punk generation, new wave, grunge etc... each one haveing their own developments, and in this list I have not even mentioned the progressive movement with groups like pink floyd, genesis, alan parson etc... The list is endless and boiling it down just one is not possibile as "Rock" is just too big a field to find one. The best however are the ones that I mentioned above. ... where do we put james brown the greatest soul man on the planet... this is exaclty what I mean.