My vote for worst, at least in the modern era: Louis Miriani, who succeeded Cobo after Cobo died in office.
Not only was Miriani heavy-handed and went against the people's wishes [[especially in his push to tear down Old City Hall: Only 21% of Detroiters wanted it razed, according to a poll at the time), he also was a crook. No text messages, but the IRS caught up to him anyway. He went to prison.