1. Emma Frost- I can't stand her. I hate the way she dresses and I hate her snobby and sarcastic attitude. Don't even get me started on the lame secondary mutation thing they had to give her so she wouldn't be completely useless if her telepathy failed her. IMO the X-Men franchise as a whole has never been as good as it was before she became an extremely prominent character because of being Cyclops' girlfriend.
2. Sentry- Did anyone like this guy? I know he is dead, but we all know how long that lasts in comics. Plus, he can still be temporarily revived.
3. Vulcan- I did not like how he was retconned into X-Men history and I did not like what that retcon did to Xavier as a character (I've disliked the big trend of demonizing Xavier who I am a fan of).
4. Fantomex- Too weird and unlikable. Terrible costume too.
5. Cassandra Nova- I definitely wasn't a fan of Morrison's X-Men run. Terrible origin (killed by Xavier in the womb, come on!), and also too weird.
In the Paul Jenkins series anyway. It was only after Bendis that people started to hate him.