What is the worst child abuse case in America?
This is a subjective thing which really cannot be answered. In a sense, every child abuse case is the worst, at least for that child.
There have been horrible real life cases like that of Debra Luptak. The Xctasy Garcia case is probably one of the worst. They poured bleach in her eyes, covered her with cigarette burns, and shattered most of the malnourished bones in her body. The thing was that Social Services did nothing. In fact, unknown to the social workers in her current state was that the mother was already on another state's child abuse registry.
There is a book called "Satan's Underground." While the veracity is uncertain in light of the "Satanic Ritual Abuse" scandals, the book has a heart-rending tale.
There have also been severe child abuse hoaxes. The Satanic Ritual Abuse (SRA) stories of the 1980's were horrible. Thankfully, they were not true for the most part. Many of the alleged acts were impossible, and there were no credible witnesses. The few witnesses that existed were subject to hypnosis or were coerced, so it seems the police created witnesses where there were none. Sadly, there were probably some copycat cases and splinter groups which did abuse in the name of "Satan," but there was no single organized ritual abuse movement as law enforcement officers and self-proclaimed experts asserted. Some did however use the controversy as a cover for criminal activities, and that is what inspired the father who put poison in the Halloween candy and blamed "Satanists."