Brainatra mentioned 5F07 "Miracle on Evergreen Terrace", and I must back up the sentiment. This episode is frustratingly, agonizingly, and insultingly bad - not just by Christmas show standards, but by the standards of "The Simpsons" as a series. The first half is actually a pretty decent character study - Bart accidentally burns down the tree and destroys the family's presents, so he covers up by fabricating a story about a burglar stealing everything. Then when all of Springfield pitches in to help the family recover from the "theft", Bart is stricken with guilt, knowing that he's basically swindled the entire town out of their money. Now, if the remainder of the episode had focused exclusively on Bart's stuggle with his conscience, then it would have been a great episode, perhaps on par with the show's first two Christmas episodes.
But that's not what happens. Instead, when Bart finally confesses, it's the entire Simpson family who receives the consequences. Homer, Marge, Lisa, and Maggie were just as much in the dark as the rest of Springfield was, yet for no apparent reason, they receive the hatred and vitriol of the townspeople for a scam in which they had no active part. Bart's self-struggle is completely abandoned in favor of a third act featuring the Simpsons becoming social outcasts, despised by everyone they know for reasons beyond their control. Finally, to "get even", the citizens of Springfield loot the Simpsons' house, stealing all of their worldly possessions. When the episode closes on that scene of the family sitting alone in the bare living room with nothing left but a single washcloth, I as a viewer do not feel entertained - I feel furious. Furious at the characters for being such heartless monsters, but even more furious at the writers for screwing up what should have been a great episode.
Give me 7G08 "Simpsons Roasting On An Open Fire" and 3F07 "Marge Be Not Proud", but this episode can rot for all I care. It's the darkest blemish on the already somewhat tarnished Season 9 - the first significant indication that "The Simpsons" would never again be as good as it was during its first eight years.