I chiefly remember Blake's 7 for acting that ranged from wooden to scenary-chewing, special effects in the best BBC tradition, and an ending that was one of the bleakest things I'd ever seen on television. I literally couldn't sleep the night after the final episode.
Of course (and I mean no disrespect to the rest of the cast) the whole thing won't work without the mighty Jacqueline Pearce, whose performance as Servalan left an indelible mark on male (and possibly female) British science fiction fans of a certain age, and the equally mighty Paul Darrow as Avon.