"Your favourite 'Folk music' instrument"

Basically, you can use any instrument you want and/or experiment as this young band are doing. There are and have, of course, been many examples over the years.


https://www.freyaraemusic.com/about

Perhaps, the question could be rephrased "Your favourite instrument which has been traditionally used for playing folk music"   or "Your favourite traditional musci instrument" or similar.
The word "folk" is actually too vague as well as all sorts of instruments are used in the wider genre...everything from Bob Dylan "going electric" and beyond. :-)

Anyway, to answer the quesion. Probably the fiddle (I play mandolin as much, of course, but it's less traditional) and The Harp. I also like various "pipes" too although I've never tried these myself.

Also, don't forget the human voice either.