Almost anytime Heffer and Filburt argue with each other. This is especially hilarious in the episode "Wacky Delly", where the two of them argue due to creative differences over the show they and Rocko are making.
And Rocko is always caught in the middle. But just takes all the bickering in strides.
In the episode "Pipe Dreams", Rocko and Heffer are trying to unclog a toilet. Cue fake commercial for a drain cleaner known as "Tropical Plumber". Heffer squeezes the bottle, and a lemon, a banana, and a pineapple start singing and dancing (Tropical Plumber, Hey! Unclog your pipes today!). They get into a canoe and are about to go down into the toilet to do their thing, when they are eaten by Spunky. The jungle disappears, and they are once again standing miserably in the flooding bathroom.
Their screaming as they got eaten was HILARIOUS.
The entirety of both "Teed Off" and "Heff in a Handbasket".
(The secret word is "pizza"!)
Peaches: Okay, an Italian dish!
Peaches: No, it's round and flat!
Peaches: No! It's flat, and has pepperoni...
Heffer: Flat calzone!
Peaches: Grr, no! Listen-
Heffer: Mean Italian guy?
Peaches: No, it's FOOD! It's round-
Heffer: I-talian donut hole!
Heffer: Hmm... Crankypants!
Heffer: A grizzly bear!
Heffer: AAAAAAAAAAAAAAAHHHHH! SEA MONKIIIIEEEEES!!!! (buzzer sounds) That was hard.
"I am the cheese! I am the best character on this show! I am better than both the salami and the bologna combined!"
During the storyboarding scene, Heffer keeps making suggestions/changes until finally...
Heffer: Do we really need the cheese at all?
Of course, Filbert was not pleased hearing that.
In "Skid Marks", Rocko is warned constantly not to get 'the fat guy'. He takes an imaginary driver's test with a pretty fat elephant, and barely manages to pass after accidentally crashing into a mutant moose (who threatened to sue them). When telling the Chameleon Brothers this, they scoff off that elephant being 'the fat guy' and point over to the REAL fat guy; a bulging pig giant stomping down the road with the flab of his back fat crushing two poor souls... No wonder no one wanted the fat guy.
"AND GOD HELP YOU IF YOU GET THE FAT GUY!"
The entirety of "Zanzibar", especially the part where they refer to themselves as a big unruly mob in song!
And they keep on singing as Rocko tries to get into City Hall, leading to this:
Guard: How do you all know the words?
Heffer: He's got you there Rock.
Rocko: I don't know the words!
Unruly Mob: (singing, standing on top of each other in a pyramid and holding sparklers) He doesn't know the words!
Also, when Rocko asks Heffer how they all know the words, and Heffer replies that they had rehearsals every Thursday, and didn't he see the flyers?
Heffer's dysfunctional family arguing in "Who's for Dinner?". In particular, Heff's dad's reactions to Peter revealing that he quit school two years ago and when he leaves the house in the cheerleader outfit.
The entirety of "Bye Bye Birdie", especially the part where Heffer dresses Filbert's bird Turdy in surfer clothes after accidentally killing him. The look on Heffer's face when he was watching Turdy's corpse on the record player playing surfer music was HILARIOUS.
Heffer is asked briefly to speak at an impromptu funeral for Filbert's dead bird. He uses a variation of the Playground Song "Beans, Beans, the Musical Fruit".
"Beans, beans, they're good for your heart. The more you you eat, the more you—"
I believe the true highlight of that episode is when he SITS on the bird, and it slowly settles into Rocko and Heffer... until they both scream and Heffer runs off with Rocko, having the bird still tied to his nose, dragged around.
How about when Rocko finally has to break the news to Filbert? He takes it very well, explaining that Turdy was a rare subspecies of bird with a short lifespan who actually lasted a long time and, even though he'll miss his pet, life goes on. Cue Heffer popping out of the house.
Heffer, cheerfully: Did you tell him I sat on it?
The scene at the end of "Canned": Rocko's former boss chews that Conglom-O gum and a beehive appears on his head and he freaks, crashing his car and starts running off to the end of the road on the screen and then he comes back running from the rabid giraffes who run right into the screen and it cuts to the two trapped flies who say "Hooray!"
The episode "Teed Off" was full of them, especially the climax when Heffer goes airborne (with a FLYING TRACTOR) with Mr. Bighead's golf ball and the dark groundskeeper has his forces unleash a full piano assault, all set to overly-dramatic World War II movie music. Then, when Heffer's "plane" get hit and plummets towards the groundskeeper's command center, everyone starts screaming in panic. And we do mean everyone:
Dark Groundskeeper: (one of his mooks leaves as his base flashes red sirens) MR. LESLIE! Get back to your post! I need a status report! Somebody tell me something!(Sees Heffer's air craft coming right towards him.) What the- AAAAAHHHH!!!!
There was also a subtle Brick Joke shown at the end. When Heffer is first seen in the episode, we pan out to what appears to be a crop circle. After the golf game, the last scene shows the aftermath golf course with a UFO and some small aliens looking at the circle.
First Nerd: You'll never be satisfied with a mortal woman, because the elf wenches are so beautiful, you'll spend the rest of your life searching for that same beauty again... (sniffs, eyes dilate)Yes.
The entirety of the Bachelor Party scene in The Big Answer, from when Heffer gets Drunk On Ice Cream to him somehow getting the plane serving as an ice cream restaurant to FLY, and finally the guys waking up the next morning after crash landing. Rocko also promises never to eat ice cream again. Remember kids, this is what a hangover feels like.
"Deat John", in which Rocko and Heffer follow instructions from a DIY book on how to fix their kitchen ...only to turn it into a bathroom. They call upon the author's services to help out, and come back later only to discover that every room has been converted into a bathroom. Yes, even the bathroom. Naturally, Rocko is enraged at first, but then comes up with the ingenious solution of throwing a bathroom party.
In "Bedfellows", Mrs. Bighead gets very excited when there is a nudist party being held at Rocko's house.
Mrs. Bighead: Oh, you're right. Oh yes, Ed, this is disgusting! Quick, Ed, get the telescope out of the hall closet!
This little bit of Black Comedy from the episode where Bev gets a nose, followed by it getting infected and needing surgery.
Dr. Hutchinson: Mr. Bighead?
Ed Bighead: Yes?
Dr. Hutchinson: I'm afraid we couldn't save her.
Ed Bighead: WHAT?!
(The color literally begins to drain out of Ed)
Dr. Hutchinson: Oh! I mean her nose! We couldn't save her nose! Ha-ha! Sorry.
(Cut to Ed as a puddle of white goo on the chair)
In "Old Fogey Froggy", the scene where a severely aged Ed bends over to pick up the soap and ends up breaking the glass with his humongous, moldy buttcheeks.
In "Wallaby on Wheels", Rocko's heart (his infatuation for a girl) and brain (logically against getting himself killed trying to go through a dangerous obstacle course) have a metaphorical fight in his mind. "Emotions win over self-preservation" in one hit.
Really Really Big Man's "Nipples of the Future" going haywire thanks to Filburt's jinx.