* The ferry bombing: This was a truly chilling act where the Joker forced two ferries full of innocent civilians to choose between blowing each other up or staying alive. This act showed the Joker's disregard for human life and his ability to manipulate even the most desperate people.
* The death of Harvey Dent: The Joker's manipulations and actions directly led to the death of Harvey Dent, a man who was once a symbol of hope and justice for Gotham. This act represented a significant loss for the city and further fueled the Joker's chaos.
* The creation of chaos and fear: The Joker's ultimate goal was to prove that even the most stable society could be plunged into chaos. He succeeded in creating a climate of fear and distrust, and his actions had lasting consequences for Gotham.
Ultimately, the worst thing the Joker did in The Dark Knight is up to individual interpretation. However, it's clear that his actions were designed to inflict maximum pain and suffering, and that he succeeded in his goal of tearing apart Gotham's social fabric.