With you on Abba, something tinny about them. I have just remembered another one for my original list.... Len Cohen.. It always sounds like him and the music have had a bad falling out 5 minutes previously and they are not going to work together.
Another vote here for Kate Bush... Can barely hear a word she is trying to sing.
I'll also add
The Bee Gees, for the reason above
Morrisey, tuneless ramblings.
jamiroqui, it's the same song over and over. Also too many tracks on his albums.
Lulu, just shouty.