Anyone who puts no heart into their music. I can't abide bland reproductions of songs that are note-correct, but lack any communication of feeling at all. The worst kind are those that are bland and not even note-correct. Blurggghghghghgh.
I can stand any (almost) amount of technical lack so long as its done from the heart. (but then I have more passion than technique, so I would say that wouldn't I?)
But I would say that its the individual renditions I dislike, and that same singer might excel in another song or genre. Sometimes a singer does something god-aweful and then goes on to produce something wonderful (and vice versa) - people change, so I'd hate to condemn anyone absolutely.