Firefly. Some of the decisions in later seasons were horrible. Like, after Book died and they revealed that his big secret was that he was God's brother. I hung around and kept watching, but when he kept appearing to the crew as a floating head in space was ridiculous.
And that episode when they all starting singing musical style when tripping balls on the poisoned steak was a bad idea. Although Niska's broadway number was memorable enough I guess.
Although nothing tops the tragic finale when River got that axe and murdered Kaylee. I mean, what was Whedon thinking? Does he get off on killing off characters or something? Frankly, the whole thing should have been left as about half a season with maybe a movie to wrap it up. That would've been a lot better.
