Ann darlin', you're close. But the most useless invention of all time is Rap Music.
Few things current or anticipated are as irritating as some black guy from the ghetto, or white guy, or Mexican guy, or Asian guy or Eskimo guy swearing at you through some sort of speaker.
Further Rap "Music" isn't music at all. It is more properly classified as "a rhythmic auditory irritant." Stores could then exercise truth in advertising having a section titled "Rhythmic Auditory Irritant Here" with a blinking arrow pointing to the correct aisles.