Note that there are at least a few Canadians in the list!

Also note that very few of the top names mentioned are those of singers who popularized folk music in the great folk scare of the 60's

Except Joan Baez & Jacqui McShee & Sandy Denny.
There are only a few early albums of Judy Collins' where she sings on key for a whole song. Well Glenn Yarbrough, Burl Ives were mentioned too.

Here's what has been forgotten or overlooked..

2 of my top favorites:
Fred Neil - mentioned by none
Maddy Prior - mentioned once

Forgotten but great voices with a large recorded discography:

A.L. Lloyd
Ewan Maccoll
Maria Muldaur
June Tabor
Tom Rush