4. Ray Riddle
The more slick the adversary, the more frustrating they can be. Ray Riddle (Fredric Lehne) was a District Deputy Chief for the Chicago Fire Department, and he had a real big problem with Chief Boden and the 51 as a whole.
He got Boden fired, and replaced him with a guy who tried to carry out his oppressive orders. Things worked out in the end, and Boden was reinstated, but the most satisfying part of this arc, which took place during season 4, is that Riddle was transferred and forced to work airports in the aftermath of the 51 drama.