The entire idea that Hobbes isn't just Calvin's imagination but actually magic.
Even sweeter when you consider that magic could've been made by Calvin's love.
The most heartwarming part of all? Per Word of God, it doesn't matter whether Hobbes is real or not—to Calvin, he's real, and that's all that matters.
There's one strip in which Calvin has been having a bad day (rain, tough classes, Moe, etc.) and then glumly walks home. As he opens the door, he's lovingly tackled and hugged by Hobbes. As he walks in with a huge smile, Calvin's mom asks if he had a good day, and he responds with, "Getting better."
A wordless Sunday strip where Calvin's dad has too much work to do to play with Calvin, then looks outside and then at his papers, and then joins his son outside to have fun in the snow. *sniffle* Not to mention the final scene from that comic ended with Calvin's dad doing his work at night, while Calvin kisses him on the head before bed. Awwwwwww. Just to top it off, Calvin's mom is lifting him up so he can reach his dad's head in the first place. The whole strip pulls it off without a single word of dialogue.
The strips right at the end and after the Raccoon Story line. This line is either this or Tear Jerker.
Calvin: I know. I'm crying because [the raccoon]'s gone out there, but he's not gone inside me.
Calvin's mom gets a nice moment in the raccoon arc. Calvin brings his mom to the raccoon, she then tells him to go back and get some towels. As she sits and waits by the raccoon's side she starts absentmindedly talking to Hobbes, who Calvin had left there to watch over the raccoon.
Calvin: I wish I had more friends, but people are such jerks. If you can just get most people to leave you alone, you're doing good. If you can find even one person you really like, you're lucky. And if that person can also stand you, you're really lucky. Hobbes: What if you find someone you can talk to while you eat apples on a bright fall morning? (which is exactly what they're doing) Calvin: Well, yeah... I suppose there's no point in getting greedy, is there?
The story line where Calvin comes home after a trip to find his house broken into and fears Hobbes has been stolen. His mom finding Hobbes:
Calvin: You're safe and sound! (sniff) And now I am, too!
And in the same comic, in the panel with the above quote, what his mom has to say about it.
Mom: It looks like we're a whole family again.
His mom and dad talking that night about how uncertain adulthood is. His mom realizes that, in the grand scheme, everything will be okay when put into perspective.
"Well, at least we weren't home when our house was broken into. No one was hurt. We're all together and okay. We lost a few of our nice things, but things don't matter much really. (snuggles up to Dad) It's hard to believe how often we forget that."
One strip has Calvin wondering why he and Hobbes dream when they sleep, afraid that their brains may get bored and they might be apart. Then Hobbes tells him the reason for their dreaming: That it doesn't have to mean being apart, because they can still play together all night in their dreams. It becomes even more powerful when you realize that Watterson wrote the strip because his cat -- the thought-model for Hobbes in many respects -- had just died, and what made him feel better was realizing that they could always be together in his dreams. Here's a panel-by-panel explanation for this event:
Calvin: [brushes his teeth while Hobbes combs his fur] I wonder why we dream when we sleep. [they then finish and head on to bed as he continues] Do our brains get bored? I wonder why we don't just plain sleep. Hobbes: I think we dream so we don't have to be apart for so long. If we're in each other's dreams, we can play together all night! Calvin: [realizes this] Hey, yeah! [shakes hands with Hobbes] Well, I'll see you in a few minutes, ol' buddy! Hobbes: I'll be there! [the lights turn off, and they both sleep as both z's appear over their heads together]
There was that one Christmas Eve strip. It was an enormous single panel with an image of Calvin and Hobbes sleeping by the fire (Hobbes sprawled out, and Calvin laying against him) and a long poem describing the surroundings from Calvin's point of view. While not overwhelmingly heartwarming on its own, the last four lines definitely qualify:
Propped against him [Hobbes] on the rug, / I give my friend a gentle hug. / Tomorrow's what I'm waiting for, / But I can wait a little more.
The Rosalyn-Plays-Calvinball arc was fun, and funny, but it was also great closure. Remember that Rosalyn is Calvin's enemy (and vice versa!) so having them make peace via the "Not So Different" trope is a wonderful way to wrap up that conflict. There's a reason it's Roz's last appearance in the strip: The conflict has been defanged and from now on she's probably going to be Calvin's favorite babysitter. But that's not so bad, is it?
The Nauseous Nocturne - A poem at the start of one of the comic anthologies. It's narrated by Calvin, speaking about his fear of a monster hiding in the dark waiting to devour him. At one point Hobbes wakes up and the monster — realizing there's a tiger with the kid it was going to eat — retreats. Calvin goes back to sleep snuggling Hobbes while finishing the poem.
''Rid of the pest, / I now can rest, / Thanks to my best friend, who saved the day.
The first Christmas strip, where Calvin looks so woobie-ish about being unable to get even a card for Hobbes.
Calvin: Uh, Hobbes?... I forgot to get you a present. I didn't even make you a card... I'm sorry, Hobbes. I didn't mean to forget.
Hobbes: It's okay, little buddy. I didn't get you anything either. (hugs Calvin) But here's a tiger hug for being my best friend.
Calvin: (smiling) Not so hard, you big sissy. You squeeze my tears out.
Hobbes: Merry Christmas.
The "Lazy Sunday" Christmas strip: "MOM! Santa didn't bring Hobbes anything!... Well, here's a present from me, anyway. *Hugs Hobbes* Hope it fits." "The best presents don't come in boxes. I'll treasure this one forever."
The "dead bird" Sunday strip◊. Watterson initially feared the comic would spark controversy, given the semi-graphic nature of the first panel; to his surprise he received several moving letters from people who had recently had loved ones pass away.
This picture◊ for a T-shirt is unusually sweet. The smile of sleeping Hobbes probably has something to do with it.
One strip featured his mom chasing him around the house, seemingly angry (which was common, as Calvin frequently fled baths, homework and his babysitter,) but ended with her massive attack of the Tickle Monster. She'd planned to tire him out and make it easier to put him to bed, but then she got carried away playing with her son. Calvin notes how her plan backfired, though, in that he was now all wound up and she needed to be put to bed.
During the baseball story arc, Calvin learns from Susie that he is the only boy who didn't sign up to play baseball during recess. After his obligatory Girls Have Cooties moment, the next strip shows him riding the see-saw with Susie, talking to her in a very non-Calvin way. Really, this more than anything emphasizes that, no matter how much they may hate each other, Calvin and Susie really are best friends.
A Sunday strip features Calvin in bed, unable to sleep. He looks over and sees Hobbes sleeping. Calvin wonders what he's dreaming about, and calls him friend, opining about how it's easier to sleep knowing someone you love is sleeping soundly next to you. With that, Calvin finally nods off.
In a story arc, Calvin makes a bet with Susie that he'll eat five worms for a nickel. Just when he's about do it, his mom shows up and drags him away. Calvin complains that she spoils everything... Until they get far enough from Susie. Then he hugs her and says "What a relief! Thanks, Mom. Great timing."
Most instances of Calvin showing actual compassion for others, such as the aforementioned raccoon and bird instances, him caring for his mom in the story arc where she's sick, and the strip where he and Hobbes plot to bulldoze a construction site and plant trees (which they don't get to as the bulldozer doesn't have its keys.) There's also an early strip where Calvin is reading about starving people in the newspaper and shows concern, saying "Some people NEVER get enough to eat." Hobbes jokes: "I know what that's like." to which Calvin responds, "No, you don't!"
During the second Duplicator arc, Calvin's good clone writing a love letter to Susie.
One beautiful moment was in an arc where Calvin had just had to deal with Moe stealing his toy truck. Calvin didn't want to steal it back, didn't want to fight Moe for it and was overall frustrated that Moe was so content to live with himself. At the end of the arc he relates the whole thing to Hobbes which ends with this short and sweet strip.
Calvin: You know, sometimes the world seems like a pretty mean place.
Hobbes: (Smiling) That's why animals are so soft and huggy.
Calvin: (As they hug each other) ...Yeah...
The famous comic of Calvin dressing as his dad is hilarious and heart warming because Calvin was able to make his mom laugh so hard she fell out of her chair.
The final line of the final strip of Calvin and Hobbes: "It's a magical world, Hobbes, ol' buddy... Let's go exploring!"
One Valentine's Day strip, Calvin gave Susie dead flowers and an insulting Valentine for a "present." She, of course, beat him up for it. But as the two walk away, Susie thinks to herself, "A Valentine and flowers! He likes me!", as Calvin thinks, "She noticed! She likes me!" No matter how mean the two are to one another, they really are just showing affection in the only way six-year-olds know how.
A dialogue-free Sunday strip involves Calvin going through his daily routine of getting ready for school with Hobbes, including breakfast and waiting out at the bus stop in the rain. Once Calvin is gone, Mom notices poor inanimate Hobbes out in the rain with an umbrella and pulls him inside. The last two panels involve Calvin looking forlornly at the classroom clock while Hobbes is looking out the bedroom window into the rain, and no words are needed to know what they're thinking.
In one Sunday Strip, Susie makes the effort to invite Calvin and Hobbes to a tea and cookie party being hosted by Mr. Bun. When Calvin told her he wouldn't have gone if she'd paid him, she looked genuinely broken up about it, and the joy on her face when he shows up anyway really shows how much she cares about Calvin (Even if she hates him a little.)
The last few strips of the Yukon arc, where Calvin's parents go out in the middle of the night to find Hobbes, as Calvin had left him outside.
In one strip, Calvin asks Hobbes if there's anything he wants for Christmas. Hobbes' reaction? "I've got a good home and a best friend. What more could a tiger want?"
A small one in an earlier strip, Calvin is heard shouting "BANG!" from off-panel before he runs up, climbs onto the armrest of Dad's chair and makes a "Kawping!" pantomime and "flies" (okay, runs) away. Dad is left confused but Calvin was shown taking the imaginary bullet to protect him.
One storyline has Calvin coming down with a bad case of the stomach flu. When his mom goes to check on him in the morning, she mentions that she's going to have to call the doctor, and Calvin won't miss school because it's Saturday. Calvin's responses? A weak "Okay" and "I know." The last panel is a visibly panicked Mom running to the phone to call the doctor. Even though Calvin normally putting up a fuss about the doctor and being sick on weekends can drive Mom crazy, she still realizes that if her son's not reacting that way, something's wrong.
Conversely, in an earlier arc when Mom got sick, Calvin offered to tell Mom a story similar to when she would tell him stories when he was sick and she preferred to get some rest instead. Saddened, Calvin remarks "It's hard to be a Mom for a Mom." before she hugged him saying he was doing just fine. Calvin, Comically Missing the Point, freaks out at the possibility that she could be contagious.
The beginning has Calvin's mom (teasingly) gushing over how cute her husband looks doing the Happy Hamster Hop for his son (although he's embarrassed by it).
In one arc, Calvin talks with Hobbes about how he accidentally went a bit to far in insulting Susie, which sent her home crying. Note that this isn't Calvin going into Never My Fault mode...he actually feels bad about it.
One strip shows Calvin in school, suddenly freaking out because he's stuck inside on a beautiful day. Miss Wormwood, rather than punishing him for it, actually displays a fair amount of understanding and sympathy for this (which makes sense, given that she's desperately waiting for retirement), and helps him calm down, giving him advice for what to do next time this happened.
This fan-made picture◊. Calvin has grown up and lets a girl play with Hobbes. It helps that the girl looks like she could be Calvin and Susie's child.
Hobbes and Bacon. It's made by Pants Are Overrated and set 26 years after Calvin and Hobbes. Part 1◊ Part 2◊ Part 3◊ Part 4◊ An extra layer of heartwarming is added to that final strip if you assume that, when Hobbes says he has a better game to play, he's referring to Calvinball. Shows that, even though he's technically not Calvin's tiger anymore, he still enjoyed all their time playing together and wants to relive those moments.
For the adults: think about that one favorite toy you had as a kid. Odds are, that toy is long gone. The idea that, almost 30 years later, Calvin STILL has Hobbes....