I love unique voices. Nearly everything mentioned here are people with voices that amaze me, because they are performers that overcame natural talents(or lack thereof) and still had successful careers. Voices that do bother me, are those of manufactured performers who aren't artists whatsoever, but just pieces placed together to target a demographic by a music marketing firm. I'll keep my Ozzy, Kurt Cobain, Janis Joplin, Billy Corgan, Neil Young, Bob Dylan, Dexter Holland, and James Hetfield's of the world. You can have the Brittany Spears, Simon Cowell projects, Lady Gaga's, Jason Mraz, Bruno Mars, Justin Beiber, Taylor Swift, and Miley Cyrus's of the world.
A "worst" performer would be that drunk guy at the karaoke bar who thinks William Hung is a cool dude.