The two sets of BFFs are good fodder for this. This troper totally cried when Blair read Serena the letter she never sent, and again when Serena urged Blair to stay in NYC after her social downfall. More recently, Chuck sells his shares in Victrola to help Nate's family. N'awwww.
The final scene between Blair and Serena in Valley Girls is probably the best one for these two yet.
And how about when Chuck's estranged father dies and Chuck spends almost two episodes in a row drowning his grief in alcohol and pot. Blair, his love-hate interest, tries to comfort him and and finally tells him that she loves him after oh so many episodes of playing cat-and-mouse with each other. But Chuck rejects her and her consoling with all his might, until the very end, when Blair finds him sitting on her bed, looking miserable and on the verge of tears. She immediately rushes to his side and lets him cry on her shoulder without a word. If you consider the amount of history between the two and what they've done to each other, this is quite the Crowning Moment of Heartwarming.
The final two minutes of season two when Chuck finally tells Blair he loves her.
Blair giving Dorota, her nanny/sidekick/housekeeper, a cell for Christmas - so they can text.
Chuck: I don't have a real mother, Blair. I never will.
Blair: That doesn't mean you're alone. I love you Chuck. And I will always be your family.
Chuck taking a drunken Dan home and putting a blanket on him, seems small but considering how Chuck used to be....
Blair and Chuck finally getting together in the season 2 finale, in the most adorable way possible.
As does their scene at the end of "O Brother, Where Bart Thou?".
Not to mention this dialogue from the following episode:
Chuck: (standing on the edge of a building) I'm Chuck Bass! Nobody cares.
Blair: I do. Don't you understand? I'll always be here. I don't want you going anywhere. I couldn't bear it. So whatever you want to do to yourself, please don't do that to me. (reaches out her hand) Please. (he takes it) Please.
Chuck: (falling into her arms) I'm sorry.
Chuck and Blair at the end of the Debarted, when she tells him how different from his father he is, and he whispers "Thank you" in her ear after she helps him up off the hospital floor.
Blair telling Chuck she will always be his family.
A small one, but still heartwarming given everything that's gone down between the three of them: in The Treasure of Serena Madre, as Chuck and Blair prepare to leave for Paris with Eleanor, they spy Nate who has just begged Serena not to leave with Trip, only to watch her go anyway. The following speaks volumes for how much both Chuck and Blair have grown:
Chuck: Do you mind if I stay?
Blair: Yes. But that's why I love you.
Blair finding out that Chuck applied to Columbia for her, knowing she was too proud to admit her failures at NYU.
The prom episode, where Chuck secretly arranges everything so that Blair's prom will be exactly like she had envisioned in her scrapbook. He also votes for her about 150 times to make sure she'll be crowned Prom Queen.
In the season two finale:
Blair: Chuck Bass... I love you. I love you so much... it consumes me. I love you.
Blair and Chuck having a rare calm moment infront of the fire. Blair is determined to believe that they're Just Friends, but then Chuck leans over (oh so gently) and kisses her. He then sweeps her off her feet and carried her up the staircase.
Blair: (after Chuck said "I love you") But, can you say it twice? No I'm serious, say it twice.
Chuck: I love you. I love you. That's three. Here's four - I love you.
When Blair finally has her chance to get approval for starting an NYU branch of her favorite secret society Chuck interrupts her meeting to deliver the news that he thinks the woman he saw at his father's grave might be his mother, despite the fact that his mother died at childbirth. Blair tells him this is crazy but when he gets ready to leave to go talk to this woman we get this exchange:
Chuck: I'll let you get back to your meeting.
Blair: Nothing is more important than being with you for this. (gets in his limo)
Carter talking to Serena about her father in the season three premiere. "Who the hell is he not to want you? I would've stayed all summer with you." Cue tremendous amounts of "awwwwwwwwww" and "squeeeeeeeeeee".
Rufus and Dan's return to the loft after Thanksgiving, where Jenny walks out into the living room, tears up the (signed) emancipation papers and tells Rufus "I don't want to not be your daughter".
Serena and Blair at the end of prom.
Blair giving Dorota a cell phone for Christmas. "So we can text."
Chuck and Lily hugging at the benefit in episode 315. Though it kind of makes you want to slap Chuck over the head for not realising who his real mother is.
The fact that Bart Bass kept a picture of Chuck as a little kid on his desk at work. Extra aww points for the fact that it was a real childhood picture of Ed Westwick.
Without Chuck encouraging Vanya to fight for Dorota and giving him some pointers on how to do it (some of which Vanya reveals were "not appropriate") the two of them probably wouldn't have gotten together, let alone married with a baby. Chuck also threw their wedding (although that was mainly to win Blair back).
Blair refusing to use the fact that Eric was at the Ostroff centre to blackmail/hurt Serena, and instead breaking him out for a night of fun.
And also much later, telling Asher and Jenny that she won't use the fact that he's gay to take them down because it would end up hurting "someone I care about". Makes you go "Awwwwwwwww" and then go "Wait, why's he trying to take her down now, along with her boyfriend who happens to have been the most supportive person about the gay thing?"
The entire Dan/Blair arc: Dan pining for her but also helping her get what she wants which is two different romantic partners and supporting her the whole way through. And then Blair finally recognizing that Dan loves her for her and them finally getting together and Blair finally calling him Dan.
The princess moment in It Girl Interrupted. Blair's upset that she never really got to be a princess, and that since her marriage to Louis is ending, she won't get to be. So Dan has her put on a big dress and buys her a tiara, and stands her on the stairs of the Met, having organized a chance for her to feel like a princess when a group of Constance girls run over and start squealing about it being Blair Waldorf, "a real life princess!"
In one of the GG blasts that begin each episode, Gossip Girl acknowledges Cece's death in season five. Also, Gossip Girl at the time was Georgina, which suggests that Cece might have become a role model to her as a wealthy, manipulative matriarch.
At the end of "Where the Vile Things Are", Blair and her mother happily collaborating together on a younger line for Waldorf Designs (B by Waldorf Designs), making the most of both of their respective strengths.
Blair insisting that Cyrus escort her alongside her father at her wedding to Louis, using his personal catchphrase.
Blair: One escort is not enough.
Chuck and Blair finally getting married in the finale.
The brief glimpses we get of their family life in the flash forward count as well. Especially Chuck lifting their son up and telling him how smart he is - a stark contrast to how Chuck himself was always belittled by Bart.
An off-camera mentioning for the boy playing Henry Bass saying that the best part of being on the show was that Ed Westwick let him buy a game on his (Westwick's) phone so he could play it between takes.