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?
Arrested Development, but it's obvious why. The first 3 seasons are perfect and then the quality suffers significantly, but that's somewhat understandable because seasons 4 onwards were filmed a decade later.
I'm of the minority opinion that season 4 is still pretty good, at least in it's original cut, but season 5 somehow manages to be a comedy show without a single funny joke in it.
I have to hand it to Season 4 of Arrested Development simply because they were on a new medium (streaming) and tried to do something different because of it. They made it work despite of the main cast not being able to shoot together for most of the shoot.