This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Gossip Girl
Blair reading her letter out loud to Serena in an early episode.
Chuck breaking apart in Blair's arms in "O Brother, Where Bart Thou?"
Chuck walking away from Blair yet again after she's professed her feelings for him in the season two finale.
The hospital scene in "the Debarted" which begins with Chuck having flashbacks of his father's death and ends with Blair helping him stand.
Chuck finally going to see Lily after she is diagnosed with cancer, and their hug as he promises he will be there every step of the way.
Blair crying alone in her room at the end of Inglorious Bassterds.
Chuck's response when Serena warns him he might lose everything.
Chuck: I already lost the only thing I cared about. They can have everything else.
The scene in season two where Blair more or less tells Chuck flat out that she loves him, and asks him to either tell her that his feelings for her are real, or admit that they are not and leave her be. He tells her that he's just playing a game with her and has no real feelings for her at all. Blair leaves and Serena, who has watched this exchange and knows Chuck feels strongly about Blair, wonders why he just said that.
Chuck: Because I love her. And I can't make her happy.
The scene at the train station in Paris, in episode 4x02. One of the most beautiful in the entire show.
Blair: I don't love you anymore.
Chuck lying on his bed and crying after finding out that Blair is pregnant with Louis' child. Bonus points for having the dog he had previously professed to hate curled up beside him.
Blair breaking down at Dorota's wedding, unable to walk her down the aisle as a part of the happy couple that blesses her marriage.