I don't really personally own anything I don't like though my wife has some stuff I don't like (Garth Brooks) and I converted some truly awful vinyl albums to MP3 for my folks that I still have because my folks didn't want them back and I haven't gotten around to getting rid of them.
If you asked my kids, they would say Shonen Knife is the worst with the cover of "Iron Man" by The Cardigans giving them their run for the money. I like to play those just to annoy the boys.
I have some stuff that I don't care for much anymore like Throwing Muses and The Jesus and Mary Chain, so I would have to say those are the worst. I used to like them back in the day, though, so I don't think they are bad.