It's not.
I guess I'll throw out my honorable mentions now. These were all episodes that I would have loved to include, but for various reasons did not. Most of these were on the short list, and barely got axed in favor of the top 20.
Number one will be forthcoming.
Treehouses of Horror II, III and IV. I think each of these has the best vignettes the series had to offer. Unfortunately, each of them also has one that just doesn't sit well with me. Burn's Frankenstein was possibly the very best one they ever did. The Monkey's paw, the Devil and Homer Simpson, Dial Z for Zombies, are all excellent. Unfortunately, just no perfect episodes in there. That's why V got the nod.
Selma's Choice Always one of my favorites. Homer vs the sandwich and Lisa drinking the blue water; both priceless moments.
Krusty Gets Kancelled Probably would have made a top 25 list.
Grampa vs. Sexual Inadequacy I think this was the last one to get cut, so it's basically #21. The subplot with the reverse vampires is what pushes it into awesome territory.
Radioactive Man Milhouse episodes were generally pretty good, and this was the best of them.
E-I-E-I-D'o I love all of the challenges, Tomacco, and particularly the B52's singing Glove Slap. Jimmy Carter accepting his challenge was a nice touch.
The Frying Game and The Old Man and the Key were both excellent episodes from season 13. Trying to weigh including some of the post-golden era episodes with any of dozens from 5-7 was tough, and a few later ones had to be excluded. Still, I love Grampa as Rebel Without a Cause and the death race with the old Mexican fellows.
Really, it's like I said earlier. The quality of shows they were cranking out from seasons 4-7 was really extraordinary. All of those were very good episodes, and plenty of them could be reasonable additions to a list such as this. That means that a whole lot of people's favorite episodes were left out. Certainly through no fault of their own. There's just not much of a point to a top 87 list, with 74 coming from 4 seasons.