How I Met Your Mother, final season.Lots of TV shows out there, good and bad. But some were great and then just took a horrible turn for no good reason, and never recovered. What's your nominee?
For me, Person of Interest has to be right up there. It was exciting, thrilling, took unexpected turns, and had great characters. They even made you care about the guest characters as much as the main ones, sometimes more so.
And then that miserable Samaritan storyline came in season three. The shows became painful to watch, and few bothered. Season four was reduced to a half season as a courtesy, and a highly unusual move. After three seasons, it's rare not to make it to five for syndication purposes. But this one had imploded so badly there was no reasonable prospect for syndication success.
I don't think of another show right now that lost its way so badly and so suddenly. Can you?
They had no major cast or creative departures to blame it on, like most good shows that suddenly go in the crapper. They just lost the ability, so consistent over the preceding 8 seasons, to tell a funny story.