I don't know that they maintain a "Top 100 List", like Billboard's, only for folk music, but they have re-released Harry Smith's "Anthology of American Folk Music", in a boxed CD set--it features 84 recordings. It is probably the most influential compilation of folk music recordings ever, because many of the 1950's and 1960's folk revivalists learned   basic repertoire and style from it. The three albums were cornerstones for most folk music record collections, and they still offer as good an introduction to variety of American folk/traditional/rural music styles as you will find--