I don't know about the worst, but one of the more disappointing was Batman: Dark Detective. After reading the awesome Batman: Strange Apparitions, I sought more stories from the same creative team. Dark Detective was a 2005 reunion of writer Steve Englehart and artist Marshall Rogers, and the story involved Bruce Wayne being reacquainted with Silver St. Cloud after many years, and the Joker running for governor. Not only was the Joker character written very over-the-top (even for the Joker), but the art from Rogers wasn't even close to his run on Detective comics back in the late 70's.