As far as least favorite characters go I think mine would be...
Imogen- I go back and forth with her. Sometimes she's okay and I like seeing her on the screen, but then there's time where they amp up the childishness in the character and she's annoying. Overall if I had to pick one of last years seniors to stay she wouldn't have been my choice, but she hasn't been too terrible this year. I would have let her have her endgame with Fiona.
Jimmy- My least favorite of the original bunch, mainly because of what his character became after the shooting. I hated how he treated Hazel and in some ways Ashley during their breakups. He wasn't a complete jerk and I like some aspects of his character but I feel like he became more of one in the later seasons. His plots were kind of a mix. Some were good (i.e. Can't Hardly Wait) while others were boring (i.e. Broken Wings). Either way I wasn't too sad that he wasn't focused on post graduation.
Mia- Liked her in season 7 when they did stories focusing on her baby daddy issues with Lucas. I mainly disliked the direction the character went in in season 8 and 9. Randomly paired with Peter, very little responsibility in raising her daughter, became a model out of the blue, etc.
Part of me wants to put Clare but I don't know. There's a lot of good times with the character and I will probably miss her once she's finally gone, but I also remember in season 11 when she was on my nerves so badly and recently she's been annoying about her Drew drama.