Halloween episodes should probably have their own category, too.
One of my favorite lines is from a Halloween episode (don’t remember which one) where Bart enters the school science fair with a ray gun that makes people dance. For some reason, Edna’s line leaves me in stitches:
“Can’t… stop… doing… the… Monkey…”.
And from the villainous Comicbook Guy story.
Lisa: I didn’t know Xena could fly.
Lucy Lawless: I told you, I’m not Xena.
Burns / The Alien: I bring you love!
** Lenny:** It’s bringing love! Don’t let it get away!
Carl: Break its legs!
– “The Springfield Files”
Not a best moment, but best name for an object: “Tactical Pants retaining System”
“I went to the McDonald’s in Shelbyville the other day.”
“Mcwhat?”
“McDonald’s restaurant. I never heard of it, either. They have over 2,000 locations in this state alone.”
“Must’ve sprung up overnight.”
“You know the funniest thing though? It’s the little things.”
“Example?”
“Well at McDonald’s you can get a Krusty Burger with Cheese. But they don’t call it a Krusty Burger with Cheese.”
“Get out! Well, what do they call it?”
“A Quarter Pounder with Cheese.”
“A Quarter Pounder with Cheese? Well, I can picture the cheese, but…do they have Krusty Partially Gelatinated Non-Dairy Gum Based Beverages?”
“They call them Shakes.”
“Shakes. You don’t know what you’re getting.”
After a deep crisis of conscience, Ned Flanders has sex with a movie star (voiced by Marisa Tomei) by a footpath in a public park. But they didn’t know there was a charity athletic event taking place that day - one of those “Run for Lupus” things. A huge crowd of people with numbers taped to their chests jogs by their supine naked bodies. Nelson is among them, and as he runs by, he points and says “Haw haw! Fight Lupus!”
Has nobody mentioned Ralphie calling Chalmers “Super Nintendo Chalmers”?
I crack up every time. I love Ralphie…
How about when Homer uses the hair-dryer to dry Bart’s tears. The visual is classic.
That reminds me of a scene from ‘Look who’s talking Too’
Mikey: “I want a Cobra Hammer-Head”
Girl (can’t remember name): “I want a Cobra Camera-Head!”
That’s not a Halloween episode, it’s the one where Bart throws Lisa’s science project - the giant tomato - at Smither’s butt, so Lisa’s new experiment is to see whether Bart is smarter than a hampster. Bart was dreaming when he uses the “dance ray” - and when he wakes up Lisa’s standing over him saying “First Prize! First Prize! First Prize!” Bart says “why are you doing that?” and Lisa says “just messin’ with your head.” 
There’s also a great visual in that episode - Bart is trying to reach the two cupcakes on top of the bookcase, mirroring the “reach towards the breasts” scene in A Clockwork Orange.
From the Halloween episode when the billboards come to life:
Wiggum: “Ah, they’re not so tough.”
“Uh, chief - you just killed the star of the High School basketball team.”
“Well, he was turning into a monster.”
The episode where Santas Little Helper runs away, after he comes home the family is petting him.
Homer: I want to pet him again!
Marge: You’ve spent five minutes petting him already. Go pet the cat.
(The cat raises her tail in expectation)
Homer (in a disgusted voice): The cat? What’s the point?
Hmm, maybe I saw it on Halloween. 
Not to cast a sour note, but it does kinda bug me that there have been five or six episodes about the dog and none about the cat. There was one that started about the cat, but at the first commercial shifted to the damn dog.
Anyhoo…
Homer: [after the dog has jumped out the car] Don’t worry. We’ll catch him, or run him over trying!
There was at least one, sorta. The one where Snowball II and a series of other cats died. Although that wasn’t exactly present.
:smack: pleasant!
Okay, I stand corrected. One episode about the cat(s).
I love cats and get sad when my cats die, but Snowball III floating face-down in the aquarium was the funniest thing I’d seen all that month.
From “Last Exit to Springfield”
BURNS
Well let’s look at the contract ourselves. (reading) Benefits? Perks? A green cookie on Saint Patrick’s Day? Ho ho, it didn’t used to be this way, Smithers. No, it didn’t used to be this way at all…
Skip back to Burn’s Atom Smashing Factory, circa 1909
BURNS’ GRANDFATHER
Come on, come on! Crack those atoms! You, turn out your pockets. (worker does so) Atoms! (counts them) One, two three, four… six of them! Take him away!
WORKER
(squeaky voice) You can’t treat the working man this way. One day, we’ll form a union and get the fair and equitable treatment we deserve! Then we’ll go too far, and get corrupt and shiftless, and the Japanese will eat us alive!
BURNS’ GRANDFATHER
The Japanese?! Those sandal-wearing goldfish-tenders? Barsh flimsha!
Back to the present day.
BURNS
If only we’d listened to that boy, instead of walling him up in the abandoned coke oven.
A great visual is from one where (I beleive) Chief Wiggum is asked to look up a law and runs across another.
Wiggum: It says here that it’s illegal to put a weasel down your pants for the purposes of gambling. (turns to his officers who are doing just that). Better cut it out boys.
Just the look of Eddie squirming around with a weasel in his pants.
Homer buys Bart a guitar which leads to Otto living with them. Near the end of the episode Homer asks Bart how the guitar lessons are coming…
Bart: I’m sorry Dad, it was too hard so I gave up. Are you mad?
Homer in understanding parent voice: No Son, of course you give up if somethings too hard. Now let’s go inside and watch TV.
This is indeed one of the funniest moments in the Simpsons, but the part that makes me roll of the floor laughing is the picuture of the ant-master, that is wearing jeans, whipping the slave and it pops up next to Kent as soon as he starts his submission speech. Every single time I see this scene it’s as funny as the first time.
Two from Mr. Burns…
Since the beginning of time, man has yearned to destroy the Sun…
If you can take advantage of a situation in some way, it is your duty as an American to do that. Why should the race always be to the swift or the Jumble to the quick-witted? Should they be allowed to win merely because of the gifts god gave them? Bah! Well, I say, cheating is the gift Man gives himself!
In the episode where Homer joins the naval reserves he causes yet another disaster at the plant, which he tries to get out of by claiming that it’s his first day. Later, he steers his submarine off course and he’s surronded by ships who think that he’s a communist.
American: Homer Simpson, you have ten seconds to explain what you’re doing.
Homer: It’s my first day. [the americans laugh]
Es mi dia primero. [the spanish laugh]
{it’s my first day in chinese} [the chinese laugh]
quack quack quack. [the penguins on the ship from Antartica chatter, then dive off of their ship]
And another from Captain McCallister:
“Yarr, this delivery of porn will keep my men straight… for about five munites!”