Axis Powers Hetalia's 3-partValentinesstrip. It has the Tsundere Germany in full deredere mode, trying to do his best to understand his partner Italy and come to terms with his feelings for him (even reading a book on Valentines and acknowledging Italy as "his first love"), and needing the help of Austria for such things. It's a big "aaaaawwwww" from the beginning to specially the end, where it's hinted that Germany might be a grown up, amnesiac Holy Roman Empire after all... and that he may have started to recover his long-lost memories..
"Come to think of it...Ever since I was born it's only been a cycle of hatred, warfare and suspicion. But now there's someone who trusts me and depends on me (even though he's often troublesome)." *Sniff*
How have we gotten this far without Holy Roman Empire's goodbye kiss to a tiny Italy? "No matter how many hundreds of years go by, I will always love you more than anyone else in the world."
There's also earlier in the strip, when Italy tells HRE not to grow into such a large empire that he ends up dying, much like Italy's own grandfather, Rome. This turns into a Crowning Moment of Heartbreaking when HRE runs off in tears without response.
The Stinger for that episode of the anime. HRE is sweeping up, using the push broom that Italy gave him. He pauses, hugs the broom, and says "It does make me happy!" D'aw!
Liechtenstein and Switzerland's interaction already was cute in the Axis Powers Hetalia webcomics. In the published manga, where we also learn their common past, it's even more heartwarming. To get it straight: Liech was a Heartwarming Orphan who almost ended up Dying Alone after a huge war that ravished her lands, then Switzerland found her and took her in even when he and his people were going through hard times. When Liech properly realized this after asking Switzerland why did he adopt her and not getting a straight answer to him, she warmly smiles and thanks him.
There's also her giving Switzerland a pair of pajamas as a gift. She accidentally gives him her own but insists that he keep them because he looks so good in them, which is adorable enough as it is. Notice, however, that when she came back to tell Switzerland her mistake, he was already wearing the pajamas. Meaning that as far as he knew, his sister gave a frilly pair of girl's pajamas, and he still wore them, because they were a gift from her. And when Liechtenstein goes to bed that night, the pajamas she's wearing are more plain, implying that she's wearing her brother's pair.
In "Happy Birthday", when England appears at America's birthday party. Though he is being his usual self, the two are a little bit less guarded. Mostly heartwarming in the Drama CD, where England wishes America a happy birthday as he (not quite yet) leaves, and the last scene with the toast. In this scene, Austria has just said he'll perform a song on his piano for a present, when England comes back.
America: What is it, England? Haven't you already left?
England: N-no, Italy found me and dragged me back here. If you don't want me here, I'll leave.
America: That's not true at all! Please stay.
England: Well, I would like to hear Austria playing the piano.
A subtler moment from Buono San Valentino. Austria was told by his current roommate Germany that there was no way he could get his ex-wife and best friend Hungary a gift, since he'd get lost in his way to a pawn shop. At the end of that part, though, a postman goes to Hungary's house and gives her a handmade gift, which "a tall guy with glasses and a lock of hair that stood up" handed him for her. Cue to Hungary smiling tenderly and cuddling the package, saying "Don't worry, I already know who he is".
After Holy Roman Empire had left, a tearful Chibitalia reports to Austria, apologizing for being late, expecting harsh punishment. However, to his surprise, Austria knew that HRE had left and allowed Chibitalia to have the day off. He and Hungary are even seen later doing Chibitalia's chores for him.
During "Russia's Big and Little Sister", we see that when they were younger and shivering from the cold, Ukraine gave her favourite scarf to Russia (of course, she wanted him to make Kiev successful...), but the fact is that since then, Russia is never seen without wearing that scarf.
In the Christmas strip,In The World, the Allies launched an attack on the Axis Powers, only for Finland Santa to arrive and deliver presents to everyone. And Germany is seen wiping his eyes, remarking how he really thought Santa wouldn't come. The last words of the strip are "As long as the countries exist such days with these people will continue to happen".
There was another strip that details the Christmas Truces, but nevertheless was quite heartwarming. Germany was musing how lonely it was on the battlefield during Christmas when England showed up, holding a soccer ball and mentioning it's still a special day even if they are on the battlefield. But what stood out the most was England's genuine smile after lying exhausted on the ground after a game with Germany.
What Switzerland tells the sleeping Liechtenstein in their Counting Sheep-CD.
Switzerland: You really do not need to be so concerned about me, Liechten. As long as you stay well and can smile everyday, that is all I ever need to stay healthy.
Just hearing Wa! Wa! World Ondo where the Axis and Allies sing together. It's so cute and happy that you cannot help but smile.
And it played at the end of The Movie, after the characters all came together and fought as one against a common enemy.
Ren Ren Renaissance also counts, as it is the only real time we see Rome and Italy consciously interacting for an extended length of time. It's really sweet to hear them laughing together and talking/singing about how much they love each other (and France-niichan, and Shinsei Roma, and Chibitalia hasn't met him yet but that other big brother too!)
Rome and Italy's entire relationship. It overlaps with Tear Jerker territory when Rome makes his brief, inexplicable return to visit a sleeping Italy — only to reveal that he's come down from Heaven to do so.
From Gakuen Hetalia: America pretending to drown (though he gets cramp and has to be rescued for real) to let England be a hero. He later genuinely asks England to forgive him for worrying him. Heck, their entire friendship event just drips of it.
Gets even sweeter when you remember something from Himaruya's notes: England can't swim. He jumped in there to save America when he himself couldn't swim. This speaks volumes about how much he cares about America below the tsuntsun.
Japan is visiting Greece, who is always flanked by stray cats. Greece says that if there is an afterlife, he would want to be reborn as a cat. Japan places cat ears on him.
All of A Bientot (Until We Meet Again) where a young girl named Lisa went on a trip to France and ends up meeting France himself. While walking in Mont Saint-Michel, France brings up the Hundred Years' War and the story of Jeanne d' Arc. Then he begins to talk about reincarnation and finishes by holding Lisa/Jeanne's hand and is just so happy that he was able to see her again.
"When I first saw you, I thought God was pulling a cruel trick on me. But I see that you're living a happy life. It seems like my wish has been granted.
Much like the one with Italy dreaming about meeting Japan as a child, this comic is a sweet moment of "Awwwww"ness that is involving young nations this time a young Liet and little Russia. It's very heartwarming indeed.
Russia: Then you'll become my friend
Liet: But...we could become friends by now...?
Russia: No, we can't! I still don't have enough strength. I'll work hard from now on.
Pretty much anytime Russia is kind towards Lithuania since he seems to be his "favorite".
Even though America ended up rejecting him, England offering to be America's friend in episode 47 of World Series was rather heartwarming especially considering that its England.
Pretty much anytime England shows a kinder side to his personality and smiles, especially noticeable when he's around America whom he may or may not have feelings for. Also, in the April Fools Day comic, England comments that America's waiter outfit looks good on him. In the Halloween 2011 comic, America calls England out specifically to dress up with him because England was the first to follow him on Twitter.
In Episode 39 of World Series after Italy modifies a car to shoot him into the sky, Germany says this:
Germany: It-it can't be... I can't believe he became one with the stars after being so trivial... [Goes on knees] He may have been a bit strange and useless... But he had his heart in the right place. If I knew this is going to happen, I would've treated him more gently... [starts sobbing] Italy... ITALLLYYYY!
Italy:[hanging from a tree] G-Germany...
Most of Germany and Italy's interactions are adorable, especially in World Series Episode 15 where Italy makes Germany lunch, complete with a note from him saying he put in all of Germany's favorite foods and Germany can't bring himself to throw it away.
Towards the climax of Paint It White, after the entire main cast falls from a spaceship (long story) and crashes into the ocean far, FAR below, we see Germany weak, injured and alone, barely managing to stand, searching the island for anyone else who made it. His strength finally gives out and he collapses, only to glimpse a glow beyond the trees - when he goes to find out what it is, Italy is sitting on the beach, making pasta over a fire. Then, all the other countries begin to emerge from the trees, each drawn by the light.
Germany: Th-that's Italy! But… how…
Japan: *walks up next to him* I think… it is the soft glow of hope.
"Italy...you did wunderbar."
This strip overlaps with Tearjerker, particularly the soldier's final words in the strip.
The story arc where England visits Japan and learns how the mythical creatures of the country are going unnoticed is a good Tear Jerker, but there's also something very sweet about England's brief friendship with the Kappa and little onion spirit. It's said multiple times in the series that the Western nations find Japan's culture to be incomprehensible and bizarre, but England still treats the Japanese mythological beings with the same respect as his own fairies, asking them to teach him a bit about Japanese ways. It's nice to see that there are ways in which even countries like England and Japan are Not So Different, and England was able to give those two a nice evening before they left the company of humans for good.
The story arc about Japan adopting various Western cultures ends with him taking in several bunnies and guinea pigs as pets. He's clearly overcome by how adorable they are, and reflects that not everything about Western culture is bad. Even the final note about pet bunnies being abandoned is made happier with a sketch that shows Japan taking in all of the stray rabbits he finds.
Episode 15 of The Beautiful World has France showing a young woman around Mont Saint Michel and telling her about the Hundred Years' War, specifically about the role that Joan of Arc played in it. He's clearly saddened when looking back upon how she was eventually burnt at the stake and abandoned by the king she'd fought for. However, he then says that he's happy to see that she's been reborn and is now able to live a happy life.
In the DVD commentary for one of the episodes, the voice actor for England mentions how this is the first big role he's gotten, as well as how this is the first time he's been asked to do a commentary. The voice actor for France congratulates him on his work and says that he's been doing a great job. For the next few seconds, the two just compliment each other on their voice acting.
Something of a Meta example, but Himuraya recently revealed England's favourite book series is Sherlock Holmes, written by Scottish author Arthur Conan Doyle. Considering the impressive amount of literature England has to choose from, it comes off like a subtle Aww, Look! They Really Do Love Each Other moment for England and his brother Scotland.
And the song itself ("My friends/Ore no tomodachi") is nothing but Sweet Dreams Fuel too. He brings up the brownie (a type of fairy known for helping around houses unseen) as being shy around everyone except England, the pixies dancing in group, and the unicorn always keeping him, England, company when he feels lonely. (Add to that, the unicorn is indeed in the coat of arms of the UK... representing Scotland. This just adds to the aforementioned Aww, Look! They Really Do Love Each Other moment with Arthur Conan Doyle.)
In a strip, Lithuania is cooking for America, who has a flashback to before the Revolutionary War. He was eating some of England's cooking and complained to himself that France's cooking was better, but upon seeing England's hopeful face, stated that it was delicious and asked for more.
In Grandpa Rome's Visit Home, Ancient Rome gets God to let him come back to Earth to see his old home, so long as Germania is there to watch him. Ancient Rome walks around modern Rome, noting that lots of buildings from his time are gone...and when he gets excited to see his old house, he finds a bench where its entrance used to be. At first Rome is sad, lamenting the fact that there are things that he should have done in life when he had the chance, and that it seems that he's been forgotten in the changing times. Germania disagrees:
Germania: You reached a splendor that could be considered unparalleled even among our own kind. People are still fascinated by the things you left behind. The ones that inherited your legacy continue to live on. You've attained the eternal peace that comes from being liberated from your boss and war. You've got nothing to regret...
England being the one to stop Canada after three hours is just icing on the cake.
Fandom example: with the Hetalia Bloodbath 2010 Christmas event, fans gathering to discuss and/or Squee! about the strips and read the updates together at the Hetalia Livejournal main community and the CAPTALIA community have formed some pretty tight bonds and become world-wide True Companions. When fans felt things beginning to come to a close, an friending spree kicked up and people started posting heartfelt goodbye notes in the comments. The event didn't end quite yet, of course, but after four threads of comments in the main comm alone and almost a week spent sleeping odd hours and hanging out together, it's not likely that anyone experiencing the Bloodbath in real time will soon forget about it.
And from the Bloodbath itself, we have Belgium giving her scarf to the shadow chasing her who later turns out to be Parallel!Spain, who calls Parallel!France #23 to say that he's fallen in love. Then, when Germany asks Italy to strip in the middle of the street so he can examine his chest and butt (no as perverted in context), Romano tries to stop Italy from doing it because "It's that perv's trap!" Italy responds "Even if it was, that doesn’t bother me at all." Then Romano saves the world. The epilogue was just one big block of these, starting with General Winter gently placing a hand on unconscious Russia's cheek, to Germany's sigh of relief when he finds Prussia and Austria asleep side by side, even Poland finding a sleeping Lithuania.
Not in the series itself, but the way the fandom is acting as a family in hard times is astonishing and beautiful. Especially this discussion.
When the earthquake in Japan happened, the fandom turned to the Hetalia Livejournal community for the latest updates on the situation and to find out who in the anime/manga community was confirmed as being alive and okay. The way EVERYONE was freaking out about wanting Himaruya to be okay, and the reaction when it was confirmed that he was...
Speaking of which, the amount of hurt-and-comfort fanarts of Kiku that arose during that time, especially the ones made with the other Nation-tans showing concern and wanting to give support. Even Belarus was drawn giving her support. Seeing the Nation-tans pulling together like that in a time of hardship is pretty sweet, even if it is just a fanwork.
Another fandom example: this map, which was created as a thank you present for Himaruya for the the 2011 Halloween event. There are over 1,000 thank you notes from fans across the globe.
The fandom has been giving thank-you presents to Himaruya for a while now after each event.