So hard to pick just one, or even ten. Off the top of my head I’d have to go with Deep Space Homer - it has one of the best ever Kent Brockman speeches (ant overlords, sugar mines - you know what I’m talkin’ about). Also, the episode with Krusty’s comeback special. And the episode where Bart takes over Kamp Krusty (Gentlemen: to evil!).
“$pringfield, or How I Learned to Stop Worrying and Love Legalized Gambling.”
So many great lines!
“I said hop in.”
“We got more gongs than the breakdancing robot that caught on fire.”
“I don’t want to alarm you, but there may be a boogyman or boogyMEN in the house.” (the delivery of this cracks me up)
“Begin the thawing of Jim Nabors!”
I must quote this episode at least once a day, when I’m around people who can appreciate it.
Some runners-up would be “Burns’ Heir,” “The Itchy and Scratchy and Poochie Show,” “Sweet Seymour Skinner’s Badassss Song,” “Homer’s Enemy,” and “Bart on the Road.”
Behind the Laughter
Jimmy Carter breakdancing: “I got a brother named Billy. And my teeth look silly. Break it down!”
Homer: “Id like to set the record straight. I thought the cop was a prostitute!”
Summer of 4’2"
Many of my favorite lines…
“I don’t know what you’ve got planned for tonight, Homer, but count me out.”
“Well, I have been eating more!”
“Sweet merciful crap! My car!”
…and a sweet ending.
Honorable mention for Lisa’s Rival…solely because of the line “Bees are on the what, now?” Without a doubt, my most used Simpsons line IRL.
“Touche, Cecil.”
I take it the various Hallowe’en episodes are out of the running.
Two Dozen and One Greyhounds never fails to amuse:
“Dogs always land on their feet.”
“Honestly, sir, you just don’t put the effort into your evil schemes anymore.”
“See my vest!”
“This is just a memory Homer. I can’t give you any new information”
The one where the cult comes to Springfield and brainwashes everyone.
“The Secterians”. Or is it “Movementarians”?
I believe it’s called The Joy Of Sect
And the group is called the Movementarians. Another classic episode.
“I am intrigued by your ideas and wish to subscribe to your newsletter.”
“Na na na na na na, Batman! I mean, Leader!”
Off topic, but is it true that Nancy Cartwright is a Scientologist?
Yes. At least according to those guys.
Not sure what the episode’s called, but the one when the Germans buy the power plant (and Homer indulges in his “land of chocolate” fantasy)
Ach du lieber! Raccoonen!
“Burns Verkaufen der Kraftwerk”
If I had to choose one episdoe, I think it’d be:
Homer Goes to College
Professor: “This proton accelerator destabilizes the atom in this chamber here, then propels it–”
Homer: “Uh, excuse me, Professor Brainiac, but I worked in a nuclear power plant for ten years, and, uh, I think I know how a proton accelerator works.”
Professor: “Well, please, come down and show us.”
Homer: “All right, I will.”
(Cut to everyone running out of the building, which is glowing green. Homer, himself glowing, casually walks out.)
Homer: (To approaching hazmat guys) “In there, guys.”
Hazmat guys: (giving a familiar wave) “Thanks, Homer.”
“Hellooo, that sounded like a pig fainting!”
And the classic, “I am so smart! I am so smart! I am so smart! I am so smart! S-M-R-T! I mean, S-M-A-R-T!”
That’s not from the Movementarian episode; that’s from the one where Burns and Homer are teamed up for that retreat in the mountains.
Bart: “Teamwork is overrated.”
Homer: “Huh?”
Bart: “Think about it. I mean, what team was Babe Ruth on? Who knows!”
Lisa+Marge: “Yankees.”
Bart: “Sharing is a bunch of bull, too. And helping others. And what’s all this crap I’ve been hearing about tolerance?”
Homer: “Hmm. Your ideas are intriguing to me, and I wish to subscribe to your newsletter.”
I have it narrowed down to two, and A Streetcar Named Marge has been mentioned already, so…
The Last Temptation of Homer - Homer almost has an affair with a co-worker at the power plant, voiced by Michelle Pfeiffer. Too many brilliant moments:
“Smithers, where does that tube go?” “Dance! Dance!”
“Get back to work, Stuart.” (said to a duck in a hardhat.)
“Homer, what’s the matter?” “Ain’t you ever seen a naked chick riding a clam before?”
“I probably shouldn’t have eaten that packet of powdered gravy I found in
the parking lot.”
“See, I got this friend named…Joey Jo-Jo…Junior…Shabadoo…” “Joey Jo-Jo, come back!”
“Think unsexy thoughts. Think unsexy thoughts.”
“Stop that! I love my wife and family. All I’m gonna use this bed for is sleeping, eating, and maybe building a little fort.”
“Fly, my pretties, fly!” “Continue the research.”
And Homer’s guardian angel taking the form of Colonel Klink. It just doesn’t get any better than that.
You Only Move Twice.
(From IMDB)
Hank Scorpio: By the way, Homer, what’s your least favorite country? Italy or France?
Homer: France.
[Scorpio adjusts a giant laser cannon pointing towards the sky]
Hank Scorpio: Heh heh heh. Nobody ever says Italy…
My sentimental favourite is the one where they go to Australia (I don’t know the official titles, I’m not cool like y’all). I’ve watched it since it was on Tracy Ullman, and I’d liked it up to then but that was the show where I said “This is fucking brilliant”. I believe the excatly moment was when they panned down passed the overworked Vishnu in the centre of the world.
My favorite episode is definitely “Homer’s Enemy,” the Frank Grimes episode. His indignation at how everything comes so easily for Homer is hilarious (and, sadly, somewhat familiar).
Grimes : “Good Heavens! Th-this is a palace! How c-- how can, how in the world can you afford to live in a house like this, Simpson?”
Homer: “I dunno. Don’t as me how the economy works.”
Grimes : “Yeah, but look at the size of this place! I… I live in a single room above a bowling alley and (muttering) below another bowling alley.”
Then, later:
Grimes : “I have had to work every day of my life and what do I have to show for it? This briefcase and this haircut. And what do you have to show for you lifetime of sloth and ignorance?”
Homer: “What?”
Grimes: “Everything! A dream house, two cars, a beautiful wife, a son who owns a factory, fancy clothes, and (sniffs) lobsters for dinner!”
The one where they go to Africa. Hi-fucking-larious.
Kitenge: He [Muntu] seized power in a bloodless coup. All smotherings.
Vicious Looking Native African #1: Look, strangers! They’re the best kind of people!
Vicious Looking Native African #2: I’ll show them the drawing I made last week.
Vicious Looking Native African #1: They’ll enjoy that.
:: The Simpsons land in a giant flower after going over Victoria Falls, and Homer rips through the flower after the petals close over them ::
Bart: How did you DO that?
Homer: It’s a flower.
Monkey in Mine #1: I found another diamond!
Monkey in Mine #2: :: slaps stone out of other monkey’s hand :: That’s a zircon, idiot.
SO MUCH FUNNY. 
Pretty much every episode mentioned in this thread – that why I love The Simpsons!
But if I had to pick one, it would be Deep Space Homer:
-
Tim Allen redux: “Oh, no, I’ve killed Wilson! Looks like it’s back to jail for me”
-
Homer’s shrewd observation that President Clinton would know where to find ‘tang’
-
The blackjack-happy NASA goon (“Where’d you get that thing?” – “Sent away”)
-
The countdown: “Three… two… one… make rocket go now”
-
James Taylor bullying Buzz Aldrin (“Listen Aldrin – I’m not as laid-back as people think – now here’s the deal: I’m gonna play, and you’re gonna float there and like it!”)
-
The NASA manager referring to the bald, 40something Taylor as an ‘unkempt youngster’
-
James Taylor boldly solving their ant problem (braving another blow from the blackjack goon [“Quiet, you!”]), then shamelessly ducking out when it backfired (“Gotta go!”)
-
The 2001 sequence with Homer and the potato chips
-
And possibly my favorite. Simpsons. moment. ever: when they’re hurtling to earth and the real astronauts are humming ‘The Battle Hymn of the Republic’ (ala The Right Stuff) and we hear what’s running through Homer’s mind: “Oh those golden grahams, oh those golden grahams, crispy crunchy graham cereal, brand-new breakfast treat”
IMO, it doesn’t get better than that.