My go to reference for this is Search Party. The first season is the most engaging TV I've ever seen, the second season is still pretty damn good, but then the show got picked up by HBO and became total shit for 2 seasons. Like god I fucking hated season 4 of that show.
As far as movies go, I gotta go with Shrek the Third. 1,2 & 4 are all great, but what the fuck was 3?
The last season of Deep Space 9 got really rough with the pah wraiths and how quickly they had to close the story. Still hit some beautiful highs, but I find how quickly they had to end Dukat’s story almost comical.