Nanoha rescuing Subaru in the first episode, which would ultimately inspire Subaru to become a mage.
Their reunion at the end of the episode counts as well. Subaru even sheds Tears of Joy when she finds out that Nanoha recognizes her.
The entire Yagami family sitting down to enjoy dinner together. Made rather bittersweet in the third Sound Stage since it's revealed that this was something the original Reinforce had dreamed of, but never got the chance to experience before her Heroic Sacrifice.
Teana's "I'm sorry, I'm sorry, I'm sorry..." as she sobs in Nanoha's arms after falsely believing that she held her back out of fear she'd surpass her, then discovering that Nanoha's real motivation was to protect her from the same hideous injuries that have befallen Nanoha herself due to constant overstraining and extreme training methods and that Nanoha had long arranged her training in the very same direction she wanted to go herself, if only she had a bit more patience. Seriously, what can be cuter than a die-hard Tsundere in a situation like this?
The morning Nanoha wakes up and notices Vivio sleeping next to her on the bed that Nanoha and Fate share. She gets up to get ready for work, and Vivio starts whimpering in her sleep when she notices that Nanoha is no longer next to her. Nanoha then picks Vivio up and places her next to a sleeping Fate. Fate then, without waking up, turns and holds Vivio. Nanoha then smiles and leaves for work.
In one episode, Nanoha encourages Vivio to stand up on her own after she trips over. In the finale, after Nanoha and Vivio have been forced to fight each other, culminating in Nanoha hitting Vivio with her strongest possible non-lethal attack, the reverted-to-normal Vivio is collapsed in the resulting crater and Nanoha tries to approach her, only for Vivio to tell her to stay back. It's a heartbreaking moment, until she continues, "I... I can stand on my own." and Nanoha realizes that, instead of being afraid of her, Vivio is trying to make her mama proud of her. Then, despite the fact that she was in immense pain and had been limping less than ten seconds earlier, Nanoha runs into the crater and grabs her daughter, tightly embracing her.
Nanoha and Subaru are trying to explain to Vivio that Nanoha is going to be her guardian, and the girl obviously doesn't get it, so Subaru says that Nanoha is going to be her mother. Vivio asks "Mama?", to which Nanoha finally says she can call her that if she wants, and then Vivio jumps into her arms and starts crying, clearly in happiness and relief as has finally found her mama.
When Fate learns that Nanoha will be Vivio's mama she notes that she's also her mama, in a way. Vivio, of course, looks pretty confused, as, like most kids, she seems to believe one should have one mama and one papa. She calls out "Nanoha-mama and... Fate-mama?", to which both Nanoha and Fate answer back, in perfect unison and the sweetest tone imaginable, "Yes?" Vivio finally gets it, smiles with still-misty eyes, and hugs them both.
Vivio sees Fate-mama's pictures of Precia, her birth mother, and Lindy, her adoptive mother. She seems a bit confused as Fate explains how one gave her life while the other raised her, but then finally pieces it together, realizes that Fate, like her, has two mothers, and gives a smile that would melt even the coldest heart.
Speaking of said pictures, there's the fact that Fate would keep a picture of Alicia and (more importantly) Precia right next to her picture of the Harlaown Family. Say what you will about Precia (and there's an awful lot to say), but you can't deny that Fate still loves her.
Vivio, in tears, tells Nanoha that she's not a real person, that Nanoha and Fate aren't her real mothers and that she deserves to die because she's a Person of Mass Destruction who only had one purpose, which was powering up a massive warship. Nanoha shakes her head and tells her that no matter how Vivio was born, she's still the little girl that Nanoha had been taking care of and the girl in front of her wasn't fake or a monster or anything like that. Nanoha then asks Vivio what she truly wanted.
Vivio:(crying her eyes out) "I... I... Nanoha-mama... I love you! I want to be with you forever! Mama... Save me!"
In a flashback in episode 22, Schach tries to talk Nanoha into giving up on the idea of finding a good mother for Vivio, knowing full well that she's the perfect candidate for that. Nanoha gives all sorts of self-deprecating reasons, including her love of flying and how she'd "already been shot down once", looking rather melancholic about it... at which point Vivio, not wanting to see her mama be sad, comes to cheer her up and pats her on the head. With one gesture, Vivio melts her (and every viewer's) heart and shoots her down again.
Nanoha: "As long as you're smiling and happy, I'll always be smiling and happy, too."
When Fate has been paralyzed by Scaglietti's Breaking Speech about how she's treating Erio and Caro just like her mother treated her, Erio and Caro counter by giving a You Are Not Alone speech about how much they love Fate for all the things she's done for them.
What really makes this scene so powerful is the buildup. Her body gives out after using every last ounce of her strength, and it seems that it just isn't enough (which would mean the death of everyone she cares about). But then Hayate and Rein show up to reveal that she actually managed to destroy the Saint's Cradle engine, effectively turning what would have been her greatest failure into a moment of triumph that her family was there to witness.
After Signum and Agito enter Unison in the end. "Agito, I feel warm when in Unison with you..." Agito doesn't answer because she is too busy wiping off sudden Well-deservedTears of Joy. She finally found her perfect Unison partner, just like Zest hoped she would. To make it even sweeter, Agito had said the same thing in the third sound stage when talking about memories of her original partner.
The manga shows how Fate met Erio, after saving him from an unethical research institute. Discarded by his parents and realizing his existence was fabricated, Erio violently lashed out at anyone who got close. Fate pays him a visit, and tries to talk with him.
Fate: "Erio, you can't just flare up at people like that..."
Erio: "SHUT UP! THIS HAS NOTHING TO DO WITH YOU!"
Fate: "It does. I want you to be happy. And I've come to take you from here."
Erio: "NO ONE ASKED YOU TO DO THAT! TO YOU, I'M NOTHING! NOTHING AT ALL!"
Fate: "You aren't nothing, I-"
Erio:*blasts Fate with electricity* "DON'T TOUCH ME!"
Fate:*gently holds Erio despite burns on her hands* "See, Erio. Right now, you're full of unhappy feelings, and thoughts of unforgivable things. I certainly wouldn't be able to understand all of it. But I want to try and understand at least a little. I think we can share some of our unhappiness. I was the same as you, Erio. Told I was unneeded by the person I loved. Told I was a failure. It was painful, and lonely. I felt like dying. But, unhappiness doesn't last forever. Fun things, happy things, if you look, you will certainly find them. I'll help you search. So please, don't hurt others in your sadness."
Likewise Fate meeting Caro for the first time is similarly heart warming. Much Like Erio, Caro is in the care of the military who note that her magic seems to be out of control and that she'll never be useful in a squad and the best she could hope for is being send out alone into battle to slaughter every enemy possible. Fate, ever so politely, tells the man relaying this information to shut up and that she will take Caro as planned. Afterwards when the two are outside in the snow, and Fate is fixing Caro's scarf into a cute little bow, the latter asks her where she should go and what she should do now. Fate's response is to ask Caro where she would like to go and what she would like to do which is something the young girl had never considered before.
The scenes of Zest, Lutecia, and Agito in the third Sound Stage. The fifth track in particular, which begins with Lutecia being adamant about reaching the next town before lunch for the sake of Agito even though she was dead tired, leading to Zest giving her a piggyback ride so she can sleep on his back while he carries the both of them there. It's just a sweet scene that shows how much the three of them care for one another, which also makes this a bit of a Tear Jerker if you listened to it after StrikerS.
Towards the end of Episode 13, there's a nice friendship moment between Hayate, Nanoha and Fate.
Hayate: "To me, youíre the people who saved my life and my beloved friends. I donít know what will happen to Section Six, but..."
Fate: "You already told us that when we joined. Both Nanoha and I chose to be here knowing that. Itís okay."
Nanoha: "Besides, getting me into the instructorsí squad and setting up Fate-chanís examinations were things you and your whole family was instrumental in."
Fate: "So now we need to help you achieve your dreams."
Hayate: "Thatís no good, weíll keep helping each other forever."
Fate: "Thatís what friends are for."
Nanoha:*salutes* "Commander Yagami, I donít think that youíre doing anything wrong right now."
Fate:*salutes* "So itís okay. Give us your orders like always with pride in your heart."
Fate calling Chrono Onii-chan. It's the only real sibling interaction between them during the season, but it's easy to tell how much Fate enjoys and values having him as a brother.
Fate: How have you been brother? Chrono:(blushing) Don't say that. We're both adults now. Fate:(smiling) That has nothing to do with us being brother and sister Chrono.
Also, the fact that Arf call Lindy mother.
Lindy's first appearance in episode 16, which basically amounted to her looking for an excuse to dote on her daughter and granddaughter.
Subaru's section in the epilogue. She's shown saving two children in danger, just as Nanoha did for her all those years ago.