Martin & Dick:

You've mentioned the Carters & I think they were wildly influential - and Dalhart before them. Huge.

But I put what I did vote for (pointlessly, I'm sure) at the BBC site.

I'm dead sure that "The Anthology" has to be it. If it's relatively small in total sales & if people keep coming up with "better examples" it must be counted as putting all the people and styles out there in one place. A masterful collection. What it did do is deeply influence the people who influenced the people who influenced the people. I can't think how many artists, song tracks, jump-off points, etc trace directly back to Harry Smith's American Folk Music.