9. Season 10
Lots of highlights here, including the wedding between Severide (Taylor Kinney) and Kidd (Miranda Rae Mayo), but the departure of Casey (Jesse Spencer) really takes the wind out of season 10’s sail halfway through.
The departure is handled well, but it takes the show a while to get its footing in the aftermath, and by the time it does we’re at the finale; where Casey comes back and reminds us all how much we missed his presence.
The fact that Jason Pelham (Brett Dalton), a well-liked addition, was little more than a placeholder for Kidd to take the lieutenant spot is also something that plays worse on repeat viewings.