8. Jack Kayson
Fans of Angel know what an effectively creepy and sinister character actor Sam Anderson can be. Despite Dr Jack's despicable ways, ER never really cashed in on Anderson's penchant for villainy. Instead, Kayson is an especially insufferable member of the board and worse still, he's somehow managed to become head of cardiology.
Devoid of any likeable qualities, the socially malignant Kayson frequently throws out questionable diagnoses when he visits the ER. Worse still, he seems incapable of accepting any questioning from the other doctors.
The peak of Kayson's unpleasantness arrives during his feud with Susan Lewis. Lewis attempts to give a patient dealing with a myocardial infarction the right treatment. Being an insufferable know-it-all, Kayson interferes and overrules Lewis' strategy, leading to the patient's death. It's one of the most infuriating moments in the show's history, the kind of thing that makes you wish a character was real just so you could punch them in the face.
Kayson's meddlesome ways pop up occasionally all the way into season 14. The amount of botched diagnoses and meddling he's guilty of is just plain criminal. While the storyline with Lewis was gripping, there didn't seem to be much else in mind for the Kayson character and his unprofessional behaviour grew to be irritating and repetitive.