The way I've heard it: The classic folk-music instrument is the fiddle, as that's easier to run across the tundra with, when the kossaks are on your heels, than a piano.

As for my own favourite folk instruments: Vox Humana, and whichever guitar I've got to hand which doesn't shred my fingertips. (I'll leave the banjo aside, much to everybody's relief.) I've acquired a four-course bouzouki recently which has a lovely sound, but I haven't tamed it yet; stay tooned.