Timothy - The Buoys. Although there may have been other songs about cannibalism, I don't believe any other one made it to the top 20 on Billboard as did Timothy. Trivia note: It was written by Rupert Holmes, famous for Escape (The Piña Colada Song) and he played piano on Timothy.
Honorable mention: Can't believe no one mentioned Bohemian Rhapsody by Queen. Very unique in its day and made it into the top 10 twice in two different decades.
Honorable mention: Can't believe no one mentioned Bohemian Rhapsody by Queen. Very unique in its day and made it into the top 10 twice in two different decades.