The end of the first episode of the first season in the anime where Kaname is complaining about Sousuke to Kyoko over the phone but ends with, "He's kind of interesting," and falls asleep with a big smile on her face.
Also the end of the first arc after Kaname thinks that Sousuke has gone but finds out he returned to continue watching over her. In Japanese, it's also when she stops calling him "Sagara-kun" and switches to First Name Basis.
Kaname giving Sousuke a haircut in episode six of Second Raid has got to count as well. Even without Wraith's meddling he was visibly nervous at the barber's shop. At Kaname's he is actually relaxed enough that he falls asleep. It's a small but profound sign of just how much trust Sousuke places in Kaname. Cue adorable blushing.
The last episode of TSR is just full of these. When Sousuke is having a severe Heroic BSOD because he thinks Kaname is dead, she shows up, ready to run into his arms, but instead beats him out it. She nearly confesses to him before convincing him to go help his comrades. Sousuke proceeds to deliver a truly incrediblesmackdown on all five of Amalgam's Venom-type Lambda Driver-equipped ASes, then disobeys direct order to meet up with Kaname again. Later, he manages to keep Mithril from firing him and get back his position as Kaname's protector, afterward apologizing to Tessa for a fight they had a while before, calling her Tessa for the first time and saying that to him she's an important person and a friend. Seeing the shocked look on her face, Sousuke assumes he was being too blunt and leaves. (She was really just stuck on him calling her Tessa.) He goes back to school and manages to stop his letter of resignation from going through. Kaname nearly beats him up for tackling their teacher (due to a laser sight courtesy of Wraith), but winds up just crying against him.
A short story is dedicated to Tessa spending some holidays at a peaceful farm in a remote location of Japan. She lives there some of the most wonderful moments in her life, and in the manga, one of the last shots of the story is herself in the middle of a large sunflower field, smiling happily.
The ending of season one of the anime. Sousuke reveals the place he had wanted to bring Kaname before a string crises diverted them, a fishing spot along the coast that no one else knows about. Even though they only have 30-40 minutes to spend there before Kaname has to go and catch a plane back to Tokyo, Sousuke is fine spending that time there with her, and asserts that he feels like he can do anything when he's with her, whether it's stop a terrorist attack or catch a fish in such a short amount of time.
In the final novel: Sosuke fulfilling the promise he made to Kaname. That promise being that the next time they meet, he would kiss her, no matter where, or who was watching.