4. Season 7
Season 7 came out swinging and never really let up over the course of its airing. There was a new boss that entered the picture, and the tension between him and the established characters added a liveliness that had been lacking in season 6. There was also a renewed sense of unity, in that all of the secondary stories coalesced nicely into the overall arc of the season.
The death of Severide’s father was handled well, as was Casey’s bout with PTSD. It all came back to the support that the 51 members had for each other, and while simple, it remains incredibly effective.