I voted active sabotage as the worst, because to me, that crosses from a 'bad work habit' into highly immoral, definitely questionable, and possibly illegal, depending on what kind of sabotage is in question.
As far as bad work habits go, I would say gossip affects the atmosphere a job the most, and can make an average job painful, and a crummy job terrible. Nobody wants to come to work, except possibly the gossipers themselves, when they know that their personal life, work habits, etc. are under discussion for no real reason other than boredom.