9. The Debarted
No show has ever suffered such an infamous decline in quality as The Simpsons (with the possible exception of Game of Thrones), but that doesn't mean that every single episode after the golden age is necessarily bad. Barthood isn't the only diamond hidden in the post-season ten rough.
The best entries that come after this are usually spoofs of some popular movie or story. Building on a framework that has already proven to be popular gives the writers of The Simpsons a better chance of putting out a decent episode. This is exactly what happened with The Debarted in season 19.
People will look past this episode because of the show's general quality around this time, but it is genuinely great. Principal Skinner and Superintendent Chalmers bring new kid Donny into the school with the sole purpose of getting a mole inside Bart's operation, sparking an elementary school version of The Departed.
The rest of the episode plays out with Bart trying to find out who the rat is, and even features The Dropkick Murphy's classic 'I'm Shipping Up To Boston' from the movie. While Homer's B story around getting a loaner car is entirely forgettable, Bart's arc is strong enough and fun enough to carry the entire entry.