The difference between a housemate ruining a series or saving/making it more interesting is whether you like them or not.
For many, Gillian McKeith and Janice Dickinson ruined their I'm a celeb series because the show was too focused on them but if you loved them, then it's fine.
Charley, Nikki, Marcus, Makosi, John James, Rex and Aaron were the most talked about housemates of their series. If you loved them, you may say they were the best, created the most storylines and had an unproportional amount of airtime due to their complex and entertaining personalities. If you didn't, you may say they ruined their series with unnecessary drama and overhyped character. It's all a matter of opinion really.
EDIT: Yes, for me Aaron has ruined this series.