Game Of Thrones was such a dip in quality it's unbeatable for the purposes of this thread. And (without giving any thing away) the seemed to be no real reason for that. Some programmes run out of budget so have to be brought to a hasty end/ an actor dies IRL requiring a quick resolution/ falling ratings mean the TV station cancels it... There are many reasons for a seemingly botched ending to a show. But in the case of Game of Thrones, none of those things seemed to be the case - it seemed to be growing in popularity, I'm sure they could have kept going much longer. Extra frustrating as the best scenes IMO were those between the battles/ set pieces - the character development with people thrown together by circumstances, the "world building" - so a final series filled with rushed resolutions to slow building plots was a real missed opportunity
Britas Empire? I must have watched dozens of episodes but have no memory of a final episode. Probably for the best, by the sound of it!
Sopranos? I liked the ending, it avoided the pitfalls I was anticipating, nicely balanced - given the years of Tony treading the line between redeemable/ villainous, I think it was only right that it finished without any moral certainty
Stranger Things? Can't complain, really. It was getting increasingly complicated by the "baddie" business (fine in S1 but more and more layers each time), but the endings for most of the main characters felt "real" given what they'd been through (one storyline was certainly a bit botched though)
The Prisoner? I hadn't realised that about the finale - having first watched it in the early nineties I assumed it must have been intended to end like that. The ending does seem a let down, but then it's hard to wrap up shows that have been based on an unresolved tension (you want him to escape each week but not if it makes the whole set up feel a waste of time)
Life On Mars? Similar, I guess, any ending had to come down one side of the tension between things like "time travel"/ "dream" . Having persevered through the American version, anticipating a carbon copy of the final UK episode, but I was glad to see they found a different spin
Lost? I only ever watched the first two episodes - after it finished I saw a lot of chatter online about it so was curious to find what the fuss was about - found a YouTube explanation and assumed it was a spoof, surely no programme could have ended up like THAT? Oh, it did ..
One not mentioned is the Netflix show You. I enjoyed the earlier seasons (for those who don't know it, it's a bit like Dexter in that you have a guy doing terrible things because he can morally justify them to himself, always just about getting away with it). But the last series felt like it was written by a very different pen, far too heavy handed and desperate to be seen to be having a morally certain conclusion, it felt like it was created by someone who hated the original series and wanted to take pleasure in spoiling the enjoyment that others had had. But not as well known as the GoT finale