Which is 'THE Greatest' is impossible to answer. But on most days, the one that tops all others to me is David Grissom in a song he did when he was with Joe Ely called "Highways and Heartaches". Amazing in every way in my book. I was already a fan of his when I first heard it, but it just reaffirmed it. It's got power, taste, soul, and THAT TONE! Whenever someone asks me about 'that Texas sound', I point them to that song (among others).
A very close second to me is Jimmy Page in "Stairway to Heaven". As far as tone, choice of notes and open spaces, the way it builds to the climax, and - most importantly IMO - how well it fits into and serves the song, that one is almost in a class by itself to me. Everything about it is just 'right'. I should add that I'm not a huge Page fan; I just think he nailed it in that one.
A close third is Leslie West in "Mississippi Queen" for many of the same reasons as I mentioned above for David and Jimmy.
Goldtop Lloyd