Well, it's not exactly folk or blues, but EVERY
acoustic guitar player should be able to play
at least two or three Eagles songs. (In fact, I think that's a law in most states.)
Also, you should learn how to play the acoustic intro to Stairway to Heaven. Preferably, you should learn to play this really, really badly. And then, every time that you go to a music store, you should play your mangled rendition of Stairway to Heaven for the amusement of the store's sales staff. They've all heard THOUSANDS of horrid versions of that song, and I just KNOW that they'd love to hear you play it. (And he laughs his evil laugh. -- He, he he!!!!)