Much as I hate to disagree with Andy Hughes, since he's the most musically knowledgeable man on the planet, the lowest woodwind instrument is the contrabassoon.
I believe we both stand corrected - I'm advised that the contrabass clarient in B flat is actually lower than the bass clarinet or the contrabassoon, unless anyone else has an input? I'm happy to be contradicted if I'm wrong - my opinions on Questions are my own, but facts stand alone.