One might argue that the greatest influence on folk music was that of the recording companies, Folkways Records, Moses Asch, Stinson, Riverside and the folk divisions of the larger recording companies, Decca (Burl Ives), RCA Victor, Columbia and later Vanguard, Elektra. I can't judge what was the most influential album on folk music itself, but the most influential on me was a single recording of 'The Martins and the Coys' played and sung by the older brother of a schoolmate in the thirties. The rhythm and lilt of the music led me to listen to folk music on the radio, notable Josef Marais' 'Sundown on the Veldt' and years later when I could afford it to purchase early albums: American Songbag, Carl Sandburg, Musicraft
             Wayfaring Stranger, Burl Ives, Decca
             Ballads of a 20th Century Minstrel, Dyer-Benet, Stinson
             Early American Folk Songs, Bob Atcher, Columbia
             Birds, Beasts, Bugs and Little Fishes, P.Seeger Folkways
             Harlem Blues, Josh White, Musicraft
             Negro Sinful Songs, Leadbelly, Musicraft
             Folk Songs and Ballads, 2 vol., Susan Reed, Victor
Most of these are 78 albums, usually four two-sided records to an album, some I have been able to transcribe to cassettes over the years but most now unplayable. I think the quest for the most influential album is like looking for the Holy Grail.