I'm going to say the second half of Game of Thrones, somewhere along S4 the show decided good writing was for chumps and opted instead to be a hollow spectacle.
So true...
I actually stopped watching it in the middle of S4 (Episode 5 or 6 I think).
My other pick is The Walking Dead. The first season was good and worthwile but after that it was a rollercoaster with all its ups and downs though the Highlights weren't that great to begin with. I stopped it last season, can't be bothered with anymore. Boring af.