I'd rather rephrase the question as 'Who has had a significant influence in folk music?' -- in which case my answer would include all those fabulous influential people who turned up in young people's lives at just the right time. And it will be different for everyone!

For myself, that would be Norman Buchan, my English teacher at secondary school, who established a Ballads Club at my Scottish school in 1957 and was influential in introducing us to the music of Jeannie Robertson, Jimmy McBeath, Pete Seeger and the Weavers, Ewan MacColl and the radio ballads, Hamish Henderson and his collecting (Lucy Stewart particularly) -- plus an awareness of the important collectors from the past.
Before I had left school I had seen concerts by the Weavers, Pete Seeger, Cisco Houston, Rambling Jack Elliot, Jeannie Robertson, Flora McNeil etc. etc. and was well aware of performers like Davy Stewart and the Stewarts of Blair.

So, my answer would have to be - anyone who has opened the ears of young people and given them a passion for this particular kind of music (which I still pursue 58 years later!).