Leverage. seriously. Leverage.
Supernatural
South Park. It only dipped down during the 2016/2017 because most TV was terrible during those 2 years, but it is terrific now with Tegridy Farms going from selling marijuana to marijuana laced with cocaine then made marijuana-free cocaine. The show started off juvenile, but over the years turned into a humorous take on lessons which turn out to be quite smart.
Not a trick question but very tricky to answer.
Obviously
Seinfeld
Friends (I never watched but based on it's response.)
Game of Thrones (Again Never watched but based on it's response from others, I Personally thought rape, dragons and incest was a stupid idea of a show.)
Those said I fail to think of any other off the top of my head, I can add more later.
Most of the time however All Shows really decline or jump the shark.
Some like Star Trek TNG kind of sucked at the beginning so any improvement based on people wanting to like the show, I think brought it to legendary status.
I think a sign a show is in decline are flashback episodes
Milli Vanilli, Bigger than Elvis
Farscape, Firefly. Only got better as time went on and managed to become movies, which is incredible, now'a days it's the exact other way around.
I'm not sure if you could have a Stargate or Farscape or Firefly in our current timeline. People take stuff a little too seriously now. Every sci fi movie they do now is like super dark, super serious, super written poorly, etc.
Bojack Horseman.
I don't watch a lot of western TV.
I feel like Breaking Bad was pretty good from start to finish. It's Always Sunny in Philadelphia has also been decent despite the absurd amount of seasons, although every season has two or three episodes that are a bit meh.
Stargate Universe ------ I loved every episode then they ended it on a cliffhanger and it makes me sad to this day
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The animated Jem and the Holograms tv series is a timeless classic to me ^_^
Lots of people are saying MASH but it took a distinct downshift once Alan Alda started directing and the episodes started to become 'look how great Hawkeye is'.