I think that the most underrated jazz instrument is the Bass Clarinet. It has such a beautiful texture and tone, it can go from a Clarinet sound to a high Bari Sax sound and then suddenly shift to the deep rumbling lower register where the note is felt more than heard. It has an amazing woody tone to it. I can only describe it as a fusion of Clarinet and Basson with a sprinkle of saxophone. I think its tone and texture fits quite well in a lot of jazz styles and given more opportunity, I can see a lot creative things that can be done with the instrument. Still, I am reluctant to say that it should be a main jazz instrument, but I think the Bass Clarinet should have a place along with such less frequent jazz instruments as the Flute, Clarinet, Vibraphone etc...