This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Young Wizards
The end of Wizards at War, where Kit sends his dog Ponch to rescue his and Nita's families and take them away from Earth, while he (and Nita) stay and expect to die fighting.
He threw his arms around the dog. One last hug, he thought. They have to let dogs into Timeheart, they have to.
Ponch is freaking out, because he knows that Kit is probably going to die, but Kit is determined to stay and fulfill his duty as a wizard to fight the Lone Power to the bitter end. He forces Ponch to go and save the two families, getting them and Ponch himself out of harm's way. Ponch has already watched at least three of their friends fall to the Lone Power in the course of the book, and was wrestling with the concept of the inherent injustice. Now he is being ordered to leave Kit, his own precious human to perish at the hands of a vengeful godlike force, so this is just unbearable to the loyal dog. His heart is breaking when Kit forces him to go, and before he leaves he lets out one last howl of mortal anguish over the horrible unfairness of it all.
"The Wizard's Dilemma."
Fred when the kids read from the Book of Night with Moon.
Deep Wizardry. Ed's sacrifice.
Shortly after Nita learns that her role in the Song is to willingly kill herself, she calls Carl for help. The resulting discussion, with Carl going over her options and thanking her for her sacrifice, on behalf of everyone who won't get killed is heartbreaking.
A Wizard's Dilemma:
Roshaun's disapperance and Dairine holding his Sunstone collar and whispering, "Where did he go?"
Ponch's sacrifice and his goodbye with Kit.
Tears of happy relief when Ronan was going to live, Roshaun still might be alive, and Kit and the sheepdog...
A Wizard Alone, as Nita struggled with her depression. But the breaking point is at the end, when she's convincing Darryl to come with her and Kit: