Dexter-First four seasons were brilliant. IMO, some of the best television ever produced, and season four saw John Lithgow firing on all cylinders as the Trinity Killer. After that: botched the Doomsday Killer story-arc, ruined characters like Deb, let Michael C. Hall run amok as director/writer/producer/head of catering/supreme overlord of the show. Plus, worst ending ever.
Game of Thrones-Season 8 was trash. There. I said it. G'head, get triggered. I relish in people trying to justify that junk. Danny's ridiculous heel turn, Dothraki hordes reappearing after getting annihilated earlier, Starbucks cups showing up on screen. Cerci and Jamie going out in the corniest fashion you can conceive. Punched 7 prior seasons of great televised fantasy in the groin. I will argue this until they shoot my remains into space to harass Martians.
Sons of Anarchy/Breaking Bad-Nothing ruins a brilliant show like making your lead characters degrade from compelling and complex anti-heroes, into jackass, bloodthirsty knobs. Walter White and Jax Teller were great characters, until the writing squad transformed them into everything you didn't want to see beloved characters become. Plus, the last seasons of both shows were disjointed, bloated, and riddled with non-compelling stories and progression.