Kira, she's horrible. All of her episodes seem to revolve around her being a religious fanatic who wants to kill the nearest cardassian. She seems to get off on being a little piece of crap to everyone too.
Jadzia Dax, I can't say that I don't like her as such, it just seems that the only point to her character (as far as season 3) is to state the obvious.
Kes. My god she was awful.
I'm not even going to count nutrek because all their characters have been written so poorly, except maybe Bones.