This surely has been a thread before, but I can’t find it. Anyhoo, for me its easily Mr. Sparkle- a wonderful tribute to all things cool about Japan, from the chat with Akira ("he identifies himself as a magnet for foodstuffs- you have very lucky dishes Mr Simpson) to the phone conversation with the America loving soap factory rep (Hello, chief. Let’s talk, why not?) to the great promotional video (I’m disrespectful to dirt. Can you see that I am serious?) and the end where we see that the soap is a joint venture of Matsumura Fishworks and Tamaribuchi Heavy Manufacturing Concern- great stuff.
I think you’ve probably picked the best one, but another personal favorite of mine is Homer finding the sugar in Lisa’s Rival. He goes insane with paranoia over this mountain of stolen sugar, guarding it with a baseball bat and giving a hilarious speech about how he’ll never be the darling of the civic fathers because of his lust for life. Also features a beekeeper who talks like Adam West, and one of my favorite Homer lines, “Bees are on the whatnow?”
This isn’t exactly a secondary plot, but I love the initial plot from Bart’s Inner Child, in which Krusty gives a trampoline to Homer. The thing turns out to be cursed - or at least it injures everyone who uses it. Homer ends up chucking it off a cliff and getting caught up in a Roadrunner parody that just slays me.
The sugar one would be my second fav, its has some great lines-
Homer: And you didn’t think I’d make any money. I found a dollar while
I was waiting for the bus.
Marge: While you were out “earning” that dollar, you lost forty dollars
by not going to work. The plant called and said if you don’t
come in tomorrow, don’t bother coming in Monday.
Homer: Woo hoo! A four-day weekend.
Homer: I found five hundred pounds of sugar in the forest that I’m going to
sell directly to the consumer! All for a low, low price of one dollar a pound.
Marge: But the grocery store sells sugar for thirty-five cents a pound.
Lisa: And it doesn’t have nails and broken glass in it.
Homer: Those are prizes! Ooh, a blasting cap.
I’m fond of the one where Bart and Lisa find the cardboard boxes, build a castle out them, survive an attack from delivery people (with dragon!), and then melt the castle with a hose. Way, way better than the primary plot in that episode.
Probably one of my most favorite visual gags was from that plot: just a simple pan over of all the kids who’ve used the trampoline. I don’t know why, but seeing all those kids lying broken and bruised on the floor like some sort of war movie had me laughing.
Maggie gets put in the Ayn Rand day care center (while Marge is rehearsing Cat on a Hot tin Roof). Maggie has to free her pacifier (in an homage to “the Great Escape” and “the Birds”). All told, best episode ever.
That was from the burning of Atlanta in “Gone With The Wind”.
Best bit from the “Sugar Baron” subplot:
That would be A Streetcar Named Desire that Marge was rehearsing.
In any case, I agree with the sugar one.
Oh, Streetcar!
Stop the Planet of the Apes! I Want To Get Off
Sugar! 
I live my life by a quote from the sugar episode, “First you get the sugar, then you get the power, then you get the women.”
I cannot think of any, but I do like a line from a critically panned episode,
Bart Simpson: Why did a vision about my future involve a story with Homer and Lincoln’s gold?
Chief: I guess the spirits thought the main vision was a little thin.
Nothing can top Mr. Sparkle, but some of your choices are very good, too. The sugar one would probably be my number two, and it has some very amusing lines, some of which were already quoted, some of which weren’t, such as:
Homer (mumbling in his sleep in some sort of foreign accent): In America, first you get the sugar, then you get the money, then you get the women.
Bart: Dad, isn’t this stealing?
Homer: Read your town charter, boy. “If foodstuff should touch the ground, said foodstuff shall be turned over to the village idiot.” Since I don’t see him around, start shoveling!
My number three would probably be Frostillicus and the Freak-E-Mart from “Lisa the Simpson.” This one just has so many amusing lines and signs: “Haunted ATM dispenses pictures of dead presidents,” “The offensive baseball cap: do you dare read it?” (“It says ‘show me your tie.’ Cute. Let’s go.”) “Gaze at the Frito found inside a bag of Doritos,” “The label fell off this can years ago. What lies inside? Treasure? Dog food?,” “Moon Pie…what a time to be alive.”
This one also deserves mention: It has some amusing lines and moments: Homer’s incredulous yell of “Tramampoline! Trabampoline!” after seeing the ad in the paper, Krusty’s plans to phase out trampoline stunts in his show in favor of dirty limericks (“There once was a man named Enos…”), and yes, the Road Runner parody. “If this was a cartoon, the cliff would break off now. [long pause] I’m thirsty! [cliff breaks off]”
The cardboard box one Miss Purl McKnittington mentioned has one of my favorite irony gags of recent seasons:
“What company are these boxes for?”
“Um…” (Notices Santa’s Little Helper eating out of trash can marked “General Industries”) “Dog, Inc.”
Some other ones that amuse me (or at least I can think of at the moment: )
-Bart and Milhouse’s factory in “Homer’s Enemy,” if only for the line “First it started to fall over…then it fell over.”
-Mr. Burns’ Howard Hughes transformation in “$pringfield”
-Mr. Burns’ attempt to become a god in “The Joy of Sect.” His rejected symbols include Mickey Mouse’s head and a special K. Mr. Burns seems to be a good source for subplots
-From “Lady Bouvier’s Lover:” Bart steals Homer’s credit card to purchase an Itchy & Scratchy animation cel guaranteed to increase in value (not a guarantee) with disappointing results. “Do you take Vysa? Oh, yes, Visa. Damn dentures.”
What about the “Badger” subplot from the area code episode? It even spawned a website. I liked that it ended without any real resolution, just Homer saying to the badger, “Go away, we’ve got bigger problems now!”
On one hand, we’ve got sugar…on the other hand, we’ve got Mr. Sparkle.
That’s a tough choice and probably worthy of GD. I’d have to lean to sugar.
Beekeeper 2: To the Beemobile!
Beekeeper 1: You mean your Chevy?
Beekeeper 2: Yes.
That foreign accent would be Italian American mangling Cuban.
I love that sugar plot!
I’ll go with the Mr. Sparkle episode, if only for Homer’s hilariously plaintive, “Why am I Mr. Sparkle?”
A question for our entire generation, I am sure.
That was critically panned? I love that one. I use lines out of that one quite frequently.
Chief: “I didn’t say to throw a firecracker in there!”
Bart: “I bought it in your parking lot.”
Chief: “That’s Crazy Talk!”
Bart: “It’s true!”
Chief": “No, that’s Crazy Talk. My brother-in-law. We’re all a little worried about him.”
Homer: “Oh, what a bleak and horrible future we live in!!”
Bart: “You mean present.”
Homer: “Right. Present. Can I get you some Soylent Green?”
Bart: “Isn’t that made of people?”
Homer: “Oh, here we go.”
Lenny: “Don’t blame me. I voted for Chastity Bono.”
Chinese diplomat: “You pay now!”
Bart: “What happened to you, China. You used to be cool.”
Chinese diplomat: “Hey! China still cool. You pay later!”
Come to think of it…none of those are from the secondary plot…
I’ll nominate the episode where Homer started his own internet company…which ultimately provoked the wrath of Bill Gates.
“Buy him out, boys!”