Supernatural's most disliked episode was, strangely, the long-running fantasy series' first backdoor pilot for a spinoff. The main Supernatural series is currently in the process of wrapping up Sam and Dean Winchester's story after an epic 15 season run. With God as the final villain, the Winchesters have their backs against the wall, and fans are eagerly waiting to discover the fates of the brothers before Jensen Ackles and Jared Padalecki move on to pastures new.
Supernatural's legion of loyal followers across the world might not feel the show's ending so keenly if there was a spinoff series currently airing that could cushion the blow, but it's not for want of trying. The CW have shown tremendous faith in Supernatural over the years and have made several attempts to capitalize on the show's fandom with a spinoff series, failing on each occasion. The first effort came with season 9's "Bloodlines" and would've moved the action from rural America to the bright lights of Chicago, where families of vampires, werewolves and other assorted monsters were involved in a mafia-style turf war. The idea was never developed into a full series, but Supernatural valiantly tried again in 2018 with "Wayward Sisters," once again hitting a dead end.
Unless Supernatural drops the ball massively within the handful of remaining episodes, the show's official lowest-rated episode, according to IMDB (via Reddit), will go down as "Bloodlines." Due to its episodic structure, Supernatural's episode ratings do have a tendency to vary wildly, but "Bloodlines" is the only episode to drop below a 6/10 score in Sam and Dean's history, coming in at a slightly-above-average 5.8. Many fans cite the distinct shift in tone for the failure of "Bloodlines," with very few elements from the main series crossing over. However, it might also be fair to say that the monster mafia concept came off a little corny, lacking the lighter edge that Sam and Dean's adventures usually employ to counterbalance the paranormal madness.
This unenviable honor is perhaps a sign of how badly misjudged "Bloodlines" was in hindsight. Not only did the episode fail to inspire calls for an entire spinoff series, but it didn't even work as a single, standalone story. The lack of focus on the central trio of Sam, Dean and Castiel can bear some responsibility, but since other Winchester-lite offerings have fared much better, the lion's share of the blame must surely rest upon the core idea of a Bloodlines spinoff. The low rating also highlights the range of more attractive options Supernatural had as potential spinoffs (such as the Ghost Facers, who enjoyed a 10 episode webseries, or Garth's werewolf family) that would've incorporated more familiar characters.
Indeed, The CW used one such idea with its second backdoor pilot, "Wayward Sisters." Based around Jodie and her band of female orphans and strays, Wayward Sisters was billed as a static spinoff that would combine the usual hunting action with the drama of a surrogate family. The "Wayward Sisters" episode enjoyed a far brighter reception than "Bloodlines," and currently holds a sold 8.4 rating on IMBD, but the option to green-light a whole series wasn't taken and, unlike before, this caused disappointment among fans. If The CW ever decide to revisit producing a spinoff when Supernatural finally comes to a close, they'd surely find a willing audience... just not with Bloodlines.
Supernatural season 15 is currently on hiatus due to the coronavirus epidemic.