Well, Clinton, we could rehash the whole lengthy "What is folk music?" discussion, I guess. But I hope most people will recognize that many songs we now accept as part of our "folklore" or "folk tradition" once had known authors (and maybe still do) and were played, sung, and even published for profit. Plus, the tunes and words to popular songs often get modified as they turn into songs the kids sing around a campfire.

As for what folk songs are most "popular" as opposed to "best known," I really think that varies a lot from region to region. "Roll On, Columbia" is probably more popular in the Pacific Northwest than in Appalachia.   Here on the west coast, I find some of John Prine's and Malvina Reynolds's or Kate Wolf's songs very popular in the folkie crowd. E.g.:
Paradise
Across The Great Divide
Give Yourself To Love
Flag Decal
Speed Of The Sound Of Loneliness
Little Boxes

But then, so are some Northeasterners' songs such as Gordon Bok's " Isle Au Haut Lullabye" and Dave Mallett's "The Garden Song."

Some of Joni Mitchell's classics are also very popular and well known, such as "Both Sides Now" and "The Circle Game."

And, although he's not "folk," a lot of "folkies" know and love many of Hank Williams's songs (Jambalaya, Your Cheatin' Heart, I'm So Lonesome I Could Cry, Hey Good Lookin', I Saw The Light, etc.).

Q: Is "country" necessarily further removed from "folk" than "popular" or "rhythm & blues" is?    (Oh, wait. I probably shouldn't open a can of worms like that in this thread.) ;-D