In "Olga Comes Home", at the end of the episode, Olga tells Helga that she's lucky that their parents don't notice her, because she's sick of "performing for them like a little wind-up doll" (as Olga describes it). Since Olga revealed that she's well-aware of Bob and Miriam's neglect of Helga, Olga's somewhat overwhelming affection and doting over her "dear baby sister" could be her reaching out to Helga and giving her the love and attention that she doesn't get from either her mom or dad. Which makes Helga's insistence that she "can't stand" Olga in a later episode really sad.
Pretty much the entirety of "Arnold's Christmas".
When Mr. Hyunh finally reunites with his long-lost daughter.
Arnold taking up the challenge of finding a person who's whereabouts have been unknown for over 20 years just to give Mr. Hyunh a happy Christmas. Keep in mind, Mr. Hyunh is not related to Arnold in any way, they just live in the same building.
How about Helga sacrificing her own gift (the Nancy Spumoni signature boots that were considered a hot Christmas item) in order to make it happen without any recognition and without any sort of notice. And keep in mind that this was from her parents who seem to have a "Well Done, Son!" Guy relationship with her. So for her to sacrifice that rare bit of recognition and love from them in order for Arnold to make Mr. Hyunh's wish come true is just...it's a mix of a Crowning Moment of Heartwarming and a Tear Jerker. And no one will ever know.
Thing is, it almost didn't happen because the guy who wanted the Snow Boots in the first place was just leaving work. Helga doesn't have any of that and convinces him to go back inside to look for Mai, because she knows that Arnold's world view is at stake of being crushed.
Helga: But if you leave now, that little football headed kid will never believe in miracles again.
And making all this even better? Helga had spent the earlier part of the episode trying to find a gift for Arnold that will "make him pine for me the way I pine for him!" It made it even more powerful when she did all this completely unknown to anyone except for the guy she convinced to keep looking.
For the short version, Helga essentially performed an Act of True Love; if there were any doubts that she really does love him, this episode will clear them all away.
A short but sweet one at the end of "Door #16". We are given an episode surrounding the mysterious Mr. Smith, a boarder who is heavily dedicated to his privacy and implied to be a government agent. Arnold is tasked with delivering a package and after going around and some chaos, we discover what the package is: A framed picture of all the boarders at the dinner table with the words "My family" written underneath.
Considering the camera position, he probably has his meals the same times they do, just him in his room.
This becomes more meaningful when Season 2 rolls around. He does not show up and Word of God says he has moved out. Having been there for three years, it's clear he grew to care for the rest of the boarders and considering the episode's timing (the middle of Season 1), perhaps he knew it was time for him to leave Sunset Arms, but wanted something to remind him of his family.
The episode "Hey Harold!", featuring quite possibly one of the most touching and adorable pairings in the history of animation, Harold and Patty.
Any episode starring Big Patty, really; how Arnold manages to convince her to fake a beatdown on Helga and they share a bit of heart to heart, how Harold stands up to Stinky and Sid when they mock Patty (twice; first time after a dance together, second time after losing an arm-wrestling match to her with pride and dignity) and even Rhonda who gave another snobby materialistic girl a verbal beatdown before things escalated into a repeat of the reform school they both went to, or worse. By the end of each episode, they all gain a bit of respect towards Patty and find that she isn't so bad as they first thought after all.
The end of "The List," where Grandma finally finds a place for her piano, on the roof, after spending all day trying to get in in the house, and sings a song to Arnold, who's had a long string of bad luck, about not letting the misfortunes of life get you down.
The end of "Sally's Comet," when their plan to work get everyone to turn out their lights works so they can see the comet in all its glory, just like Phil did with his dad seventy years ago.
Grandpa Phil/(Flashback)Phil's Dad: "Twenty Three Skidoo! Boys, you're going to remember this for the rest of your lives!"
Arnold: "So, what do you think we'll be doing in seventy years?"
Gerald: "Lookin' at this comet again!"
Phil: "Yes sir! Course, I won't be. I'll be... y'know."
The end of "The Beeper Queen". Miriam's finally taken some initiative and has proven she's more than a "smoothie"-guzzling scatterbrain by taking over for Bob at the beeper emporium, and she loves it. But once she realizes she's neglected Helga, she gives it all up to come back home, and she and Helga even hug. This being the Pataki family makes it all that more meaningful.
Also, any time Big Bob shows some Hidden Depths and love toward Helga.
The ending of "Crabby Author". "Even if you're afraid to go looking for magic, magic still may come looking for you. The end."
In the Veteran's Day episode, Gerald is eager to hear his dad tell war stories from his time in the Vietnam War, only to be disappointed when Martin reveals he was just a file clerk. He offhandedly says that the closest he got to any action was when he drove through an area where a battle had taken place and found a bleeding soldier named Miller that he made a bandage for. Later when they are visiting the Vietnam war memorial, Miller shows up and reveals that if Martin hadn't done that, he would have bled to death and he's been waiting for over twenty years to personally thank him for saving his life. Miller tells Gerald that his dad is a real hero and introduces his wife and kids to Martin and Gerald. Miller's son salutes Martin and they all watch the fireworks display together.
Also the ending to "Phoebe Skips". It shows how Helga and Phoebe`s friendship means to each other.
Helga: "You're more than just my sidekick. You're my best friend."
In "Helga on the Couch," the Flashback to how Helga and Arnold first met surely qualifies.
And is preceded by a Crowning Moment of Heartbreaking in the previous scene, where Helga is announcing that she's going to preschool, only to be completely ignored by her parents.
Dr. Bliss' entire character is extremely heartwarming — she's the kind of person a troubled kid like Helga really needs in her life, a genuinely good person who became a child psychologist because she wants to help kids. Having known Helga for barely a day, she cares more about Helga than people who have known her for her entire life, and gets her to open up not necessarily by wheedling her but by encouraging her to be better. It's telling that when Helga first walks into her session, she's bitter and combative, but then by the end she doesn't want to go, promises to come back for another session, and even runs back to give the doc a big hug.
It's a small moment, but the first time we see Helga start to bond with Dr. Bliss is when Helga discovers they both know the same jump rope rhymes. Cut to the two of them jumping rope side by side while saying the same rhyme.
"Field Trip", where Arnold and his grandma team up to free an old, ill-treated turtle from the aquarium. The episode ends with them watching him swim away to the sea, as the sun rises.
In the Thanksgiving Episode, Helga returns home - expecting her family not to have missed her. Instead, she sees Miriam demanding the police find her missing daughter, Big Bob about to get in his Hummer to patrol the city and Olga having just finished dozens of fliers she intends to hang up. Naturally, they're overjoyed to see her safe and sound, and all admit out loud that they love her.
Meanwhile, at the boarding house, Arnold returns home to gladly celebrate their tradition of Fourth of July (as Pookie flips the holidays every year). However, his grandparents and the boarders have set-up a bonafide Thanksgiving dinner for him on the roof.
In "Best Man," Coach Wittenburg asks Arnold to be just that at his wedding - saying he's his only true friend.
Also, him sacrificing the win for his wife - again.
There was an episode where the kids try to take the subway but it ends up malfunctioning leaving them alone with a homeless man (Mark Hamill) who keeps telling them to "Get out of [his] house," a blind man (voiced by Kevin Michael Richardson) with his dog, a psychologist (Tony Jay) and his patient (Kath Soucie) trying to overcome claustrophobia, a retired nurse, and an old man stuck underground. Of course tensions run high but near the end, Arnold notices there's something wrong with the blind man's dog and points it out. The retired nurse joins Arnold while the blind man sits back and worries. Some moments later he asks if his dog is all right, Arnold reassures him and tells him his dog was actually female and just gave birth to a litter of puppies. The blind man just starts crying tears of happiness and says "I'm a daddy?!" And to make it even better, the subway turns back on (thanks to Grandma) and everyone cheers!
Note that the subway starts up while everyone was singing a song initiated by the homeless man.
In "Casa Paradiso", where Grandpa tries to sell the boarding house and tries to make Arnold, who refuses to leave, come with him to live in a condo. The heartwarming part comes in as the residents stand by Arnold's protesting of selling their home, and they swear that they'll take care of him.
Arnold singing to an ill Glueboy the mule in "The Racing Mule".
Helga admitting her feelings for Arnold to Lila in "School Play", and Lila stepping down from the Juliet role to give Helga the chance to kiss Arnold, saying she finds her feelings "ever so sweet".
"Phoebe Breaks A Leg" is a true friendship episode for Helga and Phoebe. After Phoebe gets hit by a bus running errands for Helga, Helga is immediately, deeply contrite and vows to be a better friend to Phoebe. She cleans Phoebe's locker, brings her stuff to wash her hair, gets her lunch for her, etc. Phoebe begins taking advantage of Helga after Helga was treating her like staff, but then when Arnold gives her a subtle What the Hell, Hero? and Helga almost gets hit by a bus, Phoebe pushes her out of the way. The two then honestly talk about their feelings when Phoebe visits Helga in hospital. It goes to show that, inconsiderate she may be, Helga truly does have a Hidden Heart of Gold and Phoebe knows this, which is why she's such a loyal friend. Aw.
When Arnold tries to collect money to buy a ticket for Mickey Kaline's last baseball match, Grandpa reminds him about the money earned when Arnold took care of dogs. When he goes to take it out, Grandpa sneaks a bill from his own pocket into Arnold's money.
Provided, he had to retract a bit when it turned out he gave Arnold a $50 bill...
Grandpa driving Arnold to the stadium to give the baseball back to Micky Kaline was heartwarming on its own. To make it even more heartwarming, Micky Kaline decides to play catch with Arnold for a while.
A very subtle moment in the beginning of "Helga's Love Potion" has Mr. Simmons handing back the students' creative writing assignments and quietly remarking to Helga how much he's truly enjoyed getting to know her through her work. It just goes to show what an excellent, devoted teacher he really is.
In "Olga Gets Engaged", Helga was perfectly willing to let Olga throw her life away so she can marry to her fiancé, Doug, who was really a smooth-talking con-artist who has another girlfriend and is planning on cleaning Olga out for everything she has once the two get married. Helga soon realizes she can't go through with it and threatens Doug to leave before she reveals to his girlfriend that he's getting married to another woman (Olga). Doug relents, but before leaving, he asks why Helga would do this despite her resentment towards Olga. Helga's response: "When your sister's in trouble, you're supposed to do something about it. It doesn't matter if you like her or not, you're supposed to do something."
Even though Doug was indeed nothing but an unfaithful gold-digger, he does admit that while he may not have loved Olga, he did at least like her, which can be seem as uncharacteristically kind for someone like him.
What prevents "Pigeon Man" from being a complete Downer Ending is that Pigeon Man now knows that there are good people like Arnold in the world and hopes to find someone like him.
"I just hope there's another Arnold where I go next..." *sniff*
Arnold's dream sequence, where the realization sets in for both Arnold and the audience that, for all intents and purposes, Arnold's grandparents are his parents, as they have raised him and cared for him ever since he was orphaned as a child.
They're small moments, but Helga's actions to help Phil win against her father in the final challenge show how much she's willing to help the guy she loves. Never insult Arnold for being an orphan, especially not in front of her.
Continuing on Helga's character, she also shows how supportive she is to Phoebe and her family when they win. She cheers her on and even gives her a thumb's up! And Phoebe, in turn, gives one back. Aww...
The ending of "Curly's Girl", mainly because Curly gives up on the "relationship" with Rhonda because he realizes how unhappy she is, and hearing his explanation.
Curly: I just figured you were suffering, you know, a saucy specimen like you pretending to like a guy like me.
It's especially sweet that Rhonda decides to approach him later to thank him for "dumping" her. Of course the moment is ruined when Curly asks Rhonda for a lock of her hair, but still...
Rhonda: I guess you acting like a total freak sometimes is just your way of showing me how much you care. And the truth is, you're...well, you're an okay guy.
Curly: Thanks. [Pause] So how about giving me a lock of your hair to keep under my pillow?
Blink and you miss, but in "Love and Cheese", Ruth can be seen looking at Arnold and Lila and smiles at them. She may have noticed Arnold more then we realized and seemed legitimately happy for him finally finding a real girlfriend.
Also, when Arnold fails to win Lila a big stuffed Teddy Bear at a (rigged) bottle toss stand, the stand attendant gives him a little stuffed turtle as a consolation prize, meekly stating he "felt sorry for him." This small but kind gesture lets Arnold present the prize to Lila.
The ending to "Helga's Masquerade", when Helga overhears Arnold admitting that he actually kind of likes her when she's being nice.
Helga: Okay...so he doesn't like-like me. But he does like me. So, that means I'm halfway there! (dreamy sigh)
Watching Arnold take care of Helga in "Beaned" especially when he tucks her in. The only thing missing from it was a kiss goodnight.
Mr. Hyunh's hit song "The Simple Things" from his brief stint as a country singer. And true to the song, the episode has a nice moral about how just because someone is talented doesn't mean they want to be rich and famous.
A scene in "The Journal" where Phil is sitting in the front seat of his (parked) car with baby Arnold sitting in his lap. Phil is honking his car horn over and over, and every time he honks the horn he boops Arnold's nose with his other hand like Arnold's nose is the car horn.
When Oskar finally learns to read by reading a little kids' book called "Pet the Kitty." After being removed from the library, he's afraid he won't be able to read the signs to get home again, but when he finds a stray cat, he picks it up and begins reciting the book. He finally manages to read well enough to find his way home. His staying up reciting the book may have annoyed everyone else, but you could just hear how happy the guy was. Added heartwarming for the cat that got a new home.
At the end of the episode, Phoebe has a much more realistic and understated Imagine Spot with Arnold and Helga on a lighthouse, watching the sunset before holding each other's hands. Awwww.
In "The Pig War", Rex is determined to cook Abner, Arnold's pig, since Abner's the spoils of the 'war'...until Arnold tells him that Abner is his pet pig. Rex is instantly revolted at the idea of eating someone's pet, especially since he has a pet dog that he loves very much (just like Arnold loves Abner) and, as Rex later explains to his grandfather, he could never imagine himself actually eating someone/something he knows personally. Together, Rex and Arnold work together to save Abner before Rex's grandfather drops Abner into the boiling pot.
The end of "Full Moon" when Harold, Stinky, and Sid confess to Principal Wartz that they were the ones who mooned him so Arnold won't have his permanent record ruined for refusing to rat on them.
Anytime Phoebe and Gerald show an interest in each other. Their status as Helga and Arnold's Beta Couple is well-deserved.
Helga throwing the titular competition of "Spelling Bee" after overhearing that her father attempted to bribe Arnold into losing.
Leichliter reuniting with his old flame Betty at the end of "Eugene, Eugene".
In "Monkeyman!", the titular character is clearly fond of Sid, who can be very hard to likeeven by characters who call him their friend, given his horror when he finds out about Sid's mugging (and though it's not stated, would have made sure it had never happened or at the very least, didn't end so badly if he had known) and how he personally retrieves Sid's stolen boots.
"GRANDMA! GRANDPA! YOU HAVE TO SEE THIS! I FOUND A MAP!!!"
Also somewhat doubles as a tearjerker, as this line would end up being unresolved... until almost 15 years later.
In a weird way, the ending of "New Bully on the Block". Due to Wolfgang being voiced by Toran Caudell (Arnold's Season 1 voice actor) and Ludwig being voiced by Phillip Van Dyke (Arnold's Season 2 and 3 voice actor), it seems oddly touching that the episode ends with the two of them becoming friends... at the expenseof screwing over Arnold himself (and the other fourth graders). In fact the whole reason the episode was made was so they could have all three of Arnold's voice actors in the same episode and a special thanks for them is listed in the credits.