Well, to keep the negative feeling going from the Actors/Actresses You Loathe, let's delve into the realm of musicians. Now I know people like Britney Spears and TECHNICALLY Paris Hilton come to mind but try to keep it within the realm of the music you like and artist of that genre you hate.
That being said, I hate Ozzy. That's it. He is a no-talent douche who only got anywhere thanks to a combination of immense talent in first Black Sabbath and then Randy Rhoads as well as his own perversity. Far as I'm concerned, Ozzy is only and will only ever be known thanks to the talent surrounding him and his drug-induced idiocy.