In my not-so-humble personal opinion:

1) Bill Monroe. He created Bluegrass music. (Not all by his lonesome, but he did the synthesis.)

2) Bob Dylan. He created the genre of the sensitive singer-songwriter to whom rhythm, meter, melody and any real meaning in the lyrics are all irrelevant. Just 'cause I'm not crazy about the genre doesn't mean I can deny his influence.

3) The Clancys and Makem. They got through to Americans with the idea that Irish music consisted of something more than Danny Boy and Galway Bay.

4) Ian Tyson. In his first career, he and Sylvia defined duet singing in the pop-folk genre. In his second, he almost single-handedly revived the cowboy song as a living medium.

5) Doc Watson. He created acoustic flatpicking as a distinct genre. (Actually, he created it on an electric guitar, but that's not supposed to be admitted in public.)