Supernatural: 10 Perfect Episodes
7. The Monster At The End Of This Book
The Monster at the End of This Book was Supernatural's first dip into meta territory by introducing Supernatural into the Supernatural universe. This was also the first appearance of Chuck Shurley, a prophet of the Lord (later revealed as God himself). Sam and Dean found him writing Supernatural books about their lives in full detail, adding another complex layer to Supernatural's mythology.
It's hilarious to watch Sam and Dean discover online fandoms dedicated to themselves with 'Sam girls' and 'Dean girls', including slash fans who Dean himself states, "they know we're brothers, right?!" With a subtle dig at Bugs, The Monster at the End of This Book is Supernatural's first nod to the fans.
As the Winchester brothers realise they never had any free will in the first place, the episode takes a sudden and terrifying turn as they face the possibility that everything in their lives has always been predestined. It goes without saying, without this episode, others, including Don't Call Me Shurley, Fan Fiction, The Real Ghostbusters, and the entirety of season 15, would never have been possible without the introduction of Chuck.