Archer. The first three seasons were fantastic, they stumbled a bit with season four, and then the floor dropped with seasons five and six. It was as if they completely ran out of ideas and at the same time were contractually bound to give all the voice actors equal time on every episode. It becomes a game of “how can I contrive a situation that gives minor characters X, Y, and Z each their required three minutes of dialog?”
Don’t blame Life’s demise an anything other than Leno’s fucking ego. Bitch didn’t deserve the 10:00 slot. He screwed a lot of things. This fantastic drama among them.
While not necessarily “terrible,” all of the following went downhill after starting well:
Crime Story, after they killed off the bad guy and then tried to resurrect him.
Batman, almost as soon as they started the second season.
Twelve O’Clock High, after Robert Lansing got canned.
The Untouchables, after they were forced to tone down the violence.
The Avengers, after Emma Peel left the series.
Voyage to the Bottom of the Sea, after they abandoned Cold War stuff for monster-of-the-week.
Mission: Impossible, after they abandoned Cold War stuff for organized crime in the US.
It actually recovered from it and was doing good stuff.
Then came the Christmas break. When it came back, it was unwatchable.
Quincy: beautifully original spin on a police procedural for the first couple of seasons, then it progressed to a disease of the week mini-movie, and finally to Jack Klugman’s weekly sermon (see “Quincy Punk*”). You could even call TV decay “Quincification.”
Kinda sad, because they introduced Anita Gillette as his girlfriend/wife in the last season, and her talent was squandered. Makes me watch the last season, though.
*Actually, “Next Stop, Nowhere,” or whatever the punk rock episode is called is hilarious, but in a *Plan 9 *way.
Hill Street Blues, after Phil Esterhaus died. They spent a season and a half trying to get back on their feet and never did.
Even Lost in Space started well and then turned to absolute crap by the time they switched from B&W to color. The inevitable fate of all Erwin Allen series.
Another example: Time Tunnel started off presenting relatively harmless bad history and had morphed to silver-skinned aliens-of-the-week by the time it ended after only one season. 
Watched all of it. Better than seasons 2-the end. Still, really bad TV. Sad.
Dude, a slight decline, but hardly massive decay. Season 4 is less popular, but Farscape more or less rocked all the way through.
Again, I know some didn’t like it, but it was still hilarious. Tobias in his Thing costume may be may favorite comedy moment in the whole series. Decay, but hardly devastating. I like it more than S3.
Sacrilidge
MASH. Basically garbage for its last three seasons.
Buffy the Vampire Slayer. Its sixth season was when the entire writing staff forgot what characters they were writing.
I gotta disagree about “Dexter”, I liked the “Barrel Girls” arc, but, yeah, after that.
Of current shows, I nominate “Sleepy Hollow”.
Roddenberry was pushed out after the second season.
My nomination is ER. It was great or very good for much of its run, but the continual cast changes and escalating melodrama made it impossible to care about any of the characters by the end.
Speaking as someone who just started Heroes yesterday, this thread is not filling me with great confidence.
All in the Family anyone? Became less than mediocre after Mike & Gloria moved west and the kid and the Teresa moved in.
(I am including the morph into Archie Bunker’s Place).
mmm
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MASH worsened, but was by no means horrible.
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Buffy also dipped a bit in season 6, but it has two of my favs. The musical and Tabula Rasa. It’s less, but hardly a massive decay.
Yikes. Dude, I’m sorry. I’m not an overly critical guy, but it is not worth it. The show tanks worse than any other in history.
Maybe not horrible, but the writing got so spotty that the actors appeared to me at times to actually be somewhat embarrassed at having to deliver the corny jokes. Like they were phoning it in.
Big Love got much worse after the second season. I blame New Tancy.
The book “These are the Voyages” says Roddenberry had a hissy fit over not getting Monday night 8 PM (“Laugh in” was very popular) and basically shitted on people like Robert Justman, D C Fontana, Fred Freiberger, NBC and a number of unworkable scripts. One surprising person is Jerry Lewis. He had a show on at 9 PM tuesday that tanked but refused to give up the time slot without getting big money. But the show’s demise had a lot of help.
The last season is somewhat OK. If you don’t have the first season to compare it too it could even honestly be called good. Season 2 is ok. Season 3… not so much.
Dead Like Me. The show was smart, inventive, and interesting as long as Bryan Fuller was on board. But it went downhill quickly after he left.
This documentary Chaos On The Bridge is pretty good. Mainly about TNG and Gene Roddenberry, but, it details some of his struggles with TOS.
It is currently on netflix streaming…