43 years today since Blake’s 7 ended with (spoiler) the entire regular cast being mown down by the bad guys, surely one of the bleakest endings to a classic sf TV series ever.
And it's done in style; has Blake gone over to the dark side? We find out that he hasn't; Avon doesn't find out in time, and takes his vengeance for a destroyed idealism. And then they all die. Even knowing it was coming when I watched it again a few years back, I was stunned by the drama of the climax. Classic stuff.