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, screaming in horror. The jungle disappears, and they are once again standing miserably in the flooding bathroom.
The various game show parodies in "Heff in a Handbasket".
"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 driver's ed film: "The automobile: A means of transportation, or a life-threatening speed casket? You decide. This is the invention lab, where they test a lot of things that are really cool. Experts are assembling life-like dummies to test the impact of a car crash. (a scientist puts together a handful of tomatoes and draws a smiley face on one of them) You may find this experiment silly. Well just let me tell YOU something, buddy: It's NOT. Remember, this could be YOU and your family! What you are about to see is a graphic depiction... and it gets kind of messy." (the car smashes into a wall, turning the tomatoes into goo. Students are heard vomiting)
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 home movies, especially Heffer's twelfth birthday when he gets carried away trying to break open a pinata and starts attacking the guests.
The entirety of "Bye Bye Birdie", especially the part where Heffer dresses Filbert's bird Turdy in surfer clothesafter 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 a certain Playground Song:
Heffer: Beans, beans, they're good for your heart, the more you you eat, the more you—-
Filbert: That's enough!
The true highlight is probably when Heffer SITS on the bird, and it slowly settles into Rocko and Heffer into what just happened... 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.
From the bowling episode, this part makes me grin just thinking about it:
Heffer: Alright! I got a strike!
Rocko: Yes, Heffer, but unfortunately, it wasn't in our lane.
Heffer: Oh... does that matter?
In the same episode, Ed's flashback to the bowling tournament where his last ball demolishes the bowling alley without the pins falling over. At all. When he only needed one pin to win. To top all things off, his ball (or what looks like his ball) rolls in and lightly touches Ed on the leg, causing him to turn into bowling pins and then fall apart. That visual is a perfect finish to the entire scene.
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.
"Dear 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!
Then at the end of the Episode she throws an offscreen party with The Nudists.
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.
The mock funeral Rocko holds for Ed to snap him out of his depression. "Ashes to ashes, dust to dust, we stick Ed Bighead in the Earth's crust." They're about to bury him alive in a garbage can and cover him with cement before he finally snaps out of it.
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.
The moment Rocko discovers the Chameleon brothers have made the movie into an art-house film is equally priceless.
Rocko: That's my nakedness! Chuck & Leon: It's our masterpiece!
A meta example behind the creation of Dr. Hutchinson. After being told by the network that they wanted to see a positive female role model in the show, Joe Murray initially declined, but after hearing that they would probably start forcing one in anyway, he accepted the request begrudgingly. An executive wanted a lady with personality, or in the more accurate words, "with a good hook", and Joe was inspired.
Bev, on increasing productivity at Conglom-O
Bev: Well, I do have a few ideas...For example, you'd increase productivity by at least 49% if you weren't all picking your noses all the time!