How can you make a list for sitcoms and not mention How I Met Your Mother? The whole last season and especially the last episode was a complete abomination that managed to make everything before it irrelevant.
Beyond that, everything in GoT after season 4 where they steadily put the "zombies" (because really, that's what they were aiming for) front and centre as the big bad and thus making everything else irrelevant. The writing going to the dogs didn't help either while as much as people bitch about Daenerys portrayal, she has has nothing on Mr "she's my queen/I don't want it"!
Star Wars going to Disney rounds up the top 3 for me, even if Rogue One and the Mandalorian were ace.