Depends on what lens through which you're looking at this question; are we analyzing who was the least productive or who failed to meet expectations? If that's the case, there are unequivocally two different answers.
If we're looking at "least productive," you have to give the nod to guys like Max Hall and Ryan Lindley, who combined threw for less than 50%, 1 TD and 13 INTs.
If we're looking at "failed to meet expectations," your answer is either Leinart or Kolb; however, even when you compare Leinart and Kolb to others, you will still find that they were more productive than, say, Hall, Skelton, Lindley, Brown, etc.
Then again, this argument sucks and only reinforces the sentiment that this team has struggled immensely to find a steady answer at QB for literally decades. As a matter of fact, since 2002, only one QB has started Week 1 games in consecutive seasons: Kurt Warner (2008,2009). Every other year it has been a different QB.