12 Great Simpsons Episodes You'll Never Own (Thanks To Fox)
7. Treehouse Of Horror XXV
The Simpsons meet the Tracey Ullman version of themselves. Groundskeeper Willie kills Maggie (revenge from an earlier Treehouse of Horror episode). Bart and Lisa go to school... in Hell, where Bart is a straight A student. Moe goes all A Clockwork Orange in the parody segment A Clockwork Yellow.
Frankly, Treehouse of Horrox XXV was one of the best Halloween specials the Simpsons have put on in years. The three-sketch episodes have been a staple of the series since the second season, and are somewhat immune to criticism (they are not canon, and it's generally pretty entertaining just to see The Simpsons spoof various elements of the horror, fantasy, and sci-fi genres), but, to be honest, they have dragged in recent seasons.
Season 26 cured all that with this cracking episode, but unless Fox opts to put out a special release of the Halloween episodes (there have been some such themed releases in the past), it's one you'll never add to your collection.