Part 1,
Part 2, and
Part 3 of this countdown offered up the following creative mix ups.
20 - Simpsons Bible Stories - Season 10
19 - 'Round Springfield - Season 6
18 - Springfield Up - Season 18
17 - The Simpsons Spin-Off Showcase - Season 8
16 - The Simpsons 138th Episode Spectacular - Season 7
15 - Flaming Moe's - Season 3
14 - Treehouse of Horror VI - Season 7
13 - Who Shot Mr. Burns? - Seasons 6 and 7
12 - Lisa's Wedding - Season 6
11 - Bart vs. Australia - Season 6
10 - Bart of Darkness - Season 6
9 - A Streetcar Named Marge - Season 4
8 - Simpsons Roasting on an Open Fire - Season 1
7 - 24 Minutes - Season 18
6 - The Way We Was - Season 2
Now the moment you have been waiting for. I give you THE TOP 5.
5 - The Seemingly Never-Ending Story - Season 17

This recursive tale is a FILO (First In Last Out) queue lovers dream. The episode unfolds as a series of stories within stories. In the episode, Lisa tells a story about Burns, in the story, Burns tells a story about Moe which contains a story about Miss Krabappel. After Burns tale concludes, a sheep busts in with a story of his own. At the conclusion of the show, we realize that the whole episode was a tale by Bart. One of the most original story structures earns this episode a spot in the top 5.
4 - Behind the Laughter - Season 11

Inspired by VH1's Behind the Music series, this episode offered a look behind the scenes of the Simpsons series where each of the characters are just actors on the show... well actually, the Simpsons are really a family, but the show is scripted. Not only does the show deliver an excellent parody of Behind the Music (complete with commercial break teases), the episode also creates an alternate reality for the family where they experienced their own arrival as a cultural phenomenon.
3 - Trilogy of Error - Season 12

This episode is supposedly a parody of Run Lola Run. I have seen that short film, and I am not sure parody is the right word, perhaps inspired by is a better description. Lola gives three branching scenarios of the same story, and Trilogy of Error gives three perspectives on a single set of events. Its kind of a The Sound and the Fury for dummies. I guess this description so far is not too flattering, so let me explain the genius behind the episode. Each act break follows one of Bart, Homer, and Lisa through the day and a series of events. During each act, the characters paths cross, but you only see the one's perspective. Its only when you see all three acts that you can piece together the complete story. Brilliant.
2 - Treehouse of Horror - Season 2

The premise is simple enough. Put the Simpsons characters in three scary tales set out of continuity with supernatural elements. This simple idea has spawned a highly anticipated yearly tradition. Not only was this the beginning of the the Treehouse episodes, but the James Earl Jones narrated segment of The Raven is one of the all time great Simpsons tales. While the formula, like all overused formulas has become a bit stale after 17 outings, this format has provided a way for marvelous Simpsonified takes of The Shining, Classic Twilight Zone episodes: It's a Good Life and Nightmare at 20,000 feet, A Ray Bradbury short story A Sound of Thunder, and Clockstoppers as well as a political tale of Clinton vs Dole and the story of Homer selling his soul for a donut.
1 - 22 Short Films About Springfield - Season 7

Topping the list is this episode which offers glimpses of the lesser knows in Springfield through hilarious vignettes. Each story is loosely transitioned to the next through clever crossover characters. Unless I am mistaken, there are actually only 20 short films, but who's counting. My favorite of these short stories are Bumblebee Man's bad day, a discussion about what they call a Krusty Burger with Cheese in Shelbyville, and Skinner's explanation of his family recipe for grilled "steamed hams." The pace of the episode never slows up, and even without much of a plot, this episode's looks at the supporting cast of the show reveals that Springfield really is an interesting place to live.