Came to say Serenity (it's just a great end, tying up a main plot point and a small background plot point, with a fantastically emotional end) but since that is already top of the list I will go with a terrible one.

Sense8.

Started out as pretty great series, I absolutely adored the first series. I felt the quality was there even if super cheesy at parts. Then the 2nd series happened and soon after, news it was cancelled. The quality had dropped quite a lot but still not terrible. Then they were given the opportunity to do a 2 n a half hr special to tie up loose ends.

It was massively rushed, the quality was completely gone, some of the characters just acted so different from how they had been previously and some were barely in it.

To be honest, I think I am still grateful, at least we got a resolution but sometimes i remember how sucky it was, i think perhaps they should have just left it on a high (even though it had already started to falter in the 2nd series)