Journey
I can't tell if I hate Spottedleaf or hate the way she was written/portrayed. Everything she did was “wise and gentle”, as if she could do no wrong. She was beautiful and kind and all-knowing…bleh. Overall, I think she was just a boring, flat character with no dimension whatsoever.
While Onewhisker was pretty kick-butt, I absolutely hate him as Onestar. I understand that he was trying to distance himself from ThunderClan to prove that WindClan didn't need them to survive, but his character did a complete 180 practically overnight, and he went from being one of Firestar's closest friends to being a complete stick in the mud.
Also have to co-sign on what everyone has already said about Squirrelflight and Ashfur…they should have just gotten together and fallen off the face of the series.