6. Brian Kelton
John C. McGinley may be known best for his lovable turn in Scrubs, but the actor has spent most of his career playing sleazy characters in crime dramas and cop shows. His turn as mayoral candidate Brian Kelton was perfectly aligned with his talents as an actor and he created one of the show's most dangerous foes.
It wasn't just that Kelton was a guy who took credit for solving cases and making changes that he didn't, but that he was so arrogant about it. He really got enjoyment out of tricking the public, and when he was shot and killed in the season 6 finale, it was hard to feel that sad.
Chicago would have been saddled with one of the most corrupt politicians in the show's history.