The Simpsons has already made the best ending episode possible. The longest-running prime-time scripted series ever, The Simpsons is regarded simultaneously as one of the greatest shows of the 20th century and most disappointing of the 21st: the golden age of seasons 3-9 is some of the best TV ever, while the zombified version it became over the subsequent 20 years is rightly lampooned. With its 30th season in full swing and the Apu controversy highlighting just how much attitudes have changed since the show began, once again the discussion of when the show will end has sparked.
The prospect of The Simpsons ending has been raised dependably throughout almost its entire run. What goes up, must come down (except for Bob Hope, Sinatra, Dean Martin and Tom Jones), and critics were always anticipating the yellow implosion. This is doubly true given that previous adult animation benchmark, The Flintstones, bowed out after six seasons. But even with a massive drop in quality, a movie, and contract renegotiations from stars demanding a pay increase, The Simpsons has soldiered on. At this point, it would take the departure of one of the main cast to halt the show (and, given how Harry Shearer's potential exit in 2015 led to no course alterations, even that may not be enough).
Whenever The Simpsons does end, though, it will be at a severe disadvantage for one simple reason. The promise of a finale will likely bring back many fans burned over the past two decades to see how the show concludes, and yet, that ideal series finale of The Simpsons already exists, buried in the middle of season 23: "Holidays of Future Passed", a Christmas-themed future half-hour first aired on December 11, 2011. Starting with the present day Simpsons family at Thanksgiving taking their annual Christmas card photo, we jump forward 20 years through a still image montage to a downbeat future: Bart is now a divorced father of two; Lisa unfulfilled in a marriage with Milhouse and daughter who resents her; and Maggie a world-famous singer.
With a strict focus on the core family and the inter-personal relationships and set against the backdrop of the festive season, this immediately hearkens back to the show's first episode, "Simpsons Roasting On An Open Fire". Indeed, it should be clarified that this episode was created as a potential series finale during the 2011 contract negotiations: showrunner Al Jean commissioned it as a back-up in case the main cast did leave. And while, as released, it was just another Christmas special, but the intentions of what make a great ending to The Simpsons remain.
Canonically, it's the truest future of The Simpsons; not a premonition or vision like "Lisa's Wedding", "Bart to the Future" or "Future-Drama", this is presented as the genuine continuation, with subsequent future episodes having attempted somewhat to fit within this continuity. But whereas "Barthood", an effective riff on Boyhood from a few years later, is certainly emotional, what makes "Holidays of Future Passed" so impressive is its sense of resolution. All the core family relationships in the show are explored and, as best they can be, ended: Homer accepts Grandpa does love him; Bart sees how Homer hides his love and failures; Lisa consolidates being a meddling mother herself. The Simpsons isn't a continuous narrative, but if it was, this would be the ideal endpoint.
The episode is still "Zombie Simpsons" and as such some of the jokes come across as broad (while a gag about Sharia law in Dearborn was criticized for being needlessly pointed). That being said, it's a lot more in-line with the golden age of the show than what it was released around, with a focus on character-logical bits meaning most gags fit what classic fans are used to. Most supporting characters get at least a nod too, making this one of the most representative outings outside of The Simpsons Movie.
The Simpsons' 645 episodes are littered with potentially great finales - "Behind the Laughter" in season 12 felt like an acknowledgment that The Simpsons had grown so big the only way to progress was to satirize itself - but "Holidays of Future Passed" is the one that most delivers a sense of actual resolution. Whenever and however The Simpsons does end (and it bears repeating that even ending the show wouldn't end the brand), it's going to struggle to match the resonance of that.