As a guitar teacher I am often asked this question. It is difficult to say, as it largely depends on how you define ‘best’. Are we talking technical ability or musical ability? Just because someone can play all the scales at a breakneck speed, can they combine them into a song? And will that song be any good? How do you define ‘good’? Most popular? Is popularity a measure of goodness?
A criticism often levelled at classical musicians is that they are technically excellent, but they are really just robots with no emotion and they do not know how to compose or improvise. And yet a relative learner does not know ‘rules’ and techniques and just goes with the feeling – after all music is meant to stimulate emotions.
For instance, many mentioned on here (Vai, Malmsteem etc) are technically unbelievable but are in no way as popular as Kurt Cobain or Noel Gallagher who are (relatively) awful technically but are immensely popular.
So what about the most influential guitarist, then? It depends on how far you want to go back. Non-classical guitar started from the African slaves in the southern USA, but it was a while before it became mainstream. From the old blues players like John Lee Hooker and Robert Johnson, certain Anglo-Saxons risked the accusations of playing “Black Man’s Music” and took it to a wider audience: we get Buddy Holly, Scotty Moore (Elvis’ guitarist), Bill Haley. Onwards and more mainstream: Eric Clapton, George Harrison, Keith Richards.