This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Homeward Bound: The Incredible Journey
Chance's sad little "Oh no, not this" as they watch the family leave. He already knows what it's like because he's already been abandoned.
When Peter tells Shadow to stay at the ranch, and he has to sit there and watch as they leave. And he repeats, "...Peter...wait...stay..."
To make it worse, the dog actually starts to whine.
Then Kate carries Sassy into the house and calls for Chance and Shadow...and Shadow is just sitting there in the sunset, looking at the spot where Peter left.
Kate comes home and discovers that her friends' pets escaped while she was supposed to be watching them. A few minutes later, she's on the phone, obviously teary, as she tries to get help from the rangers in the area and just gets told that it's very unlikely that simple pets could survive in the wild.
After the family is told that Shadow, Chance, and Sassy ran away, Peter proposes the idea of going to look for them, and Bob has to tell them that they "have to be realistic" about the possibility that they won't find them. Peter, already unhappy about Bob becoming part of the family and the changes he's put the rest of them through, lashes out at him. As a child, the viewer's probably more focused on the unhappiness the children are going through in this scene, but as they get older, they realize it couldn't have been easy for Bob either.
The point when Sassy falls into the river. The panic and desperation in her voice as she desperately tries to keep her head above water and how the dogs run to save her breaks my heart. Especially when Shadow is devastated when he fails.
After a while of waiting up for Sassy, Shadow assumes she's dead, and says "I'll miss you, Sassy." All Chance can do is look on sadly.
When Sassy's been rescued by that kind soul that's drying her, she looks like she's been through the wringer, and she's so out of it that all she does is mutter "Shadow. Chance. Shadow."
What about little Molly, who was lost in the woods? All she could do is cry how much she missed her parents, who were in tears while searching for her.
Near the end, when Shadow has fallen in the muddy pit, is injured, and can't quite get out even with Chance and Sassy encouraging him. *sob*
Especially since Shadow was willing to give up because he feels he's too old. Just seeing him so broken after being so determined to reach Peter is just heartbreaking.
Also, he tells Chance and Sassy to leave him. Chance tries to get him to keep going like he did during the journey, and tells him that he loves him.
Chance: Look, you pushed me this far, now I'm pushing you the rest of the way. You know, back in the woods when things were really bad, I always believed we'd make it because I thought you were too stubborn to quit! And you're not going to quit, not now, not when we're this close! Now try again!
Shadow: Do you think it's easy for me to admit I can't do it? I'm too old...
Chance: That's not true! There's nothing you can't do.
Shadow: I have nothing more to give, Chance. And it's time for you to be on your own.
The last scene. Peter thinks that Shadow didn't make it, after Chance and Sassy come back. He turns, brokenhearted, to go back inside... and Shadow slowly limps up the hill, and then he sees his boy... and runs to him.
Peter turns away. "He was old. It was too far. He was just too old." Shadow - old, worn-out, injured, limping Shadow - finally tops that last hill and just the emotion in his voice... "Peter." Cue wah-wahs.