There is no one greatest guitarist. Is there one greatest folk singer or player?

Everyone has something unique to contribute and although you can suggest your preferences, to state that one is superior to another without accompanying criteria
is silly.

Anyone who plays guitar exceptionally well regardless of style or field, as most real musicians will tell you "is the greatest!"

Here's a funny criteria for you. For accompanying simple but pretty folk songs, you can't beat Burl Ives. His um plunk supported every song that he ever did and could elicit encores a-plenty for his playing and tasteful guitar backup.

Barney Kessel's accompaniment for Julie London's recordings is masterful, notably
"Cry Me A River".

Eric Weissberg's accompaniment for Judy Collins' "Suzanne" is also unique.

Peggy Seeger did a lovely guitar accompaniment for "I Wish I Was a Single Girl Again" on Folksongs of Courting and Complaint.

I think that Bruce Langhorne's accompaniments for Odetta on Tradition Records were excellent.

Hey, it's all apples and oranges here. Greatest? Depends on what we're talking about.