Harry Smith's Anthology of American Folk Music as a nucleus. Then—if you want to go back to the early precursors of the folk music revival in the United States, you should look into the recordings of some of the earlier "interpreters" (prior to the Kingston Trio's 1958 hit single of Tom Dooley).
        Probably, at the time, the best known of the early interpreters was Burl Ives (long before his Little Bitty Tear and Little White Duck aberrations). Simple, straightforward renditions of folk songs. Ives was one of the first singers of folk songs that most of the general public was acquainted with as early as the Forties. Early 10" LPs on Decca.
        A female singer of folk songs reasonable well-known that early was Susan Reed. She accompanied herself with Irish harp and something she couldn't put a name to, but she said it resembled a zither. Sweet voice, good songs. She starred in a movie (1948, I think) about a young girl from the Appalachians brought to New York by an entrepreneur to sing in night clubs. Lots of singing in that movie.   
        Richard Dyer-Bennet, classically trained tenor and classic guitarist, sang folk songs in the ancient minstrel tradition, not making any attempts to sound "folk." He felt that the songs themselves deserved the best that he could give them. Many folkies don't like his classical approach, but no comprehensive collection would be complete without a few samples of his work.
        Two singers who grew up in the folk tradition are Jean Ritchie and John Jacob Niles. Jean Ritchie is about as clean, honest, and straightforward a singer of folk songs as you will ever hear (and she occasionally drops in on Mudcat). Must have. John Jacob Niles is rather controversial. On any given song in his repertoire, he couldn't seem to make up his mind whether he wrote it or collected it. Serious folklorists regard him with extreme suspicion. To say his style of singing is "unique" is an understatement, but he certainly has a way with a song (occasionally his style gets in the way of a song). Highly dramatic. You should at least hear him.
        Frank Warner collected and sang songs. He learned Tom Dooley from Frank Profitt and the Kingston Trio learned it from Frank Warner's record. Cynthia Gooding, multi-lingual, big voice. Good songs well sung. Along the same line is actor Theodore Bikel. He and Cynthia Gooding did an Elektra album together called Young Man and a Maid. If you find it, grab it. Among groups, "The Weavers" is (are?) a must. And recordings of Pete Seeger. Joan Baez goes without saying. Odetta. Josh White. Harry Belafonte's highly accessible "big" arrangements.
        Many won't be too thrilled by my list because, obviously, not all of the ones I mention above are "pure" folk singers. But they are some of the interpreters who brought folk music to the general public and who were largely responsible for sparking the popular folk music revival that, in the U. S., can be said to have started with the KT's 1958 recording. That inspired similar groups and spin-offs such as the Limeliters, the New Christy Minstrels, et al, and the pop-folk thing was off and running.
        Plunge into the Smithsonian-Folkways catalog and swim aroung. And spend your children's inheritance on Folk-Legacy recordings.
        I'm leaving a lot out, but many are listed above by others. I haven't even mentioned singers like Ewan MacColl and Peggy Seeger. Or A. L. Lloyd. Or Jean Redpath. Or. . . .

Good hunting.

Don Firth