Apart from the human voice, I agree it has to be the fiddle, which certainly has been the most significant folk instrument. Whether you play one or not, a lot of the time you are playing what started out as fiddle tunes, or singing songs to original 16 bar fiddle tunes. Guitars and squeeze boxes are more versatile of course, since you can provide some accompaniment at the same time, notwithstanding arguments about chords and harmony being modern affectations.