>>119565590the Stewie Kills Lois/Lois Kills Stewie two parter for Family Guy but without the "it was all just a simulation" cop out ending.
The biggest plot thread throughout the show, up to that point, would've been tied up in a neat and exciting way, it used the massive changes in the status quo to take many of the characters in interesting directions, it included some level of actual character development, and it had some memorable comedic and (at least in the DVD version) musical moments.
Ending the show there would've also allowed the show to end at around 100 episodes while planting seeds for a sequel series that took the show in meaningful directions. Finally, ending the show there would've avoided the show much of the seasonal rot it did go through in our timeline.