Any "wet-tuned" instrument. I.e.some accordions and melodeons. That slightly off-key sound is just horrible! I play B/C button accordion myself - nice pure sound if I hit the right notes!
Musical saw is even fine by me, so long as it's not a fiddle being sawed.
Very happy with hurdy-gurdies, nyckelharpas, autoharps, bagpipes (outdoors if it's the GHB), melodeons in pure-tone, bodhrans, spoons, washboards, string basses, and banjos - if played well and at an appropriate owing in the session.
I once went to a concert of multiple serpents and ophicleides: now that was an odd experience, but not unlikeable, if a bit off-key at times.
But as others have said, it's not so much about the instrument as how it's played.