Season 9, Episode 7: "Bad Boys"
By season nine, Supernatural's focus had shifted to the ongoing war between heaven and hell, leaving less time for small but powerful episodes like "Bad Boys." When Dean gets a call from a man from his past, the boys travel to a reform school in New York where Dean spent a few months when he was younger. The episode combines a spooky monster of the week (a little boy's dead mother is slowly going mad as she tethers herself to Earth to protect her son) with a deep dive into a previously unexplored period of Dean's life.
"Bad Boys" reveals that Dean had a chance to thrive in a life that didn't involve hunting, and knowing that was ripped away from him gives this installment the extra punch it needs to be a standout.