9. All Singing, All Dancing
As the quality of the show declined over the years, fans would eventually come to lower their expectations significantly when tuning into The Simpsons. That's what makes episodes like "All Singing, All Dancing" even more of a black eye, stuck as they were in the middle of what was still considered a Golden Age for the show.
To be fair, nobody likes network-mandated clip shows that recycle old footage in place of an actual plot. Not the cast, not the crew, not the fans. And while "All Singing, All Dancing" certainly wasn't the first offender, taking place in the ninth season, it's possibly the most strikingly pointless; particularly thin on plot and rewatchability.
With the family holed up at home having rented a movie that turns out to be a musical, they regale memories (actually, bewilderingly, home videotapes) of times that they also sang songs in the past. Also, Snake turns up with a gun, then leaves after they stop singing. That's it. That's the entire plot.
It's certainly no enviable task to try and craft some semblance of narrative around a mandatory cost-cutting exercise, but who needs to sit through a small smattering of songs when they were all widely available, even in the late 90s?
Not to mention, a Greatest Hits of Simpsons songs featuring neither Union Strike Folk Song NOR Dr. Zaius? For shame.