This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Stand by Me
Chris' reveal of his past to Gordie, and Gordie's own breakdown after they find Ray Brower's body.
The scene with the body, for that matter.
"The kid wasn't sick. The kid wasn't sleeping. The kid was dead."
The reason the story is being told, and the fates of Gordie's friends, with the final narration revealing how Teddy never got to live his dream of joining the Army and Chris' life came to sudden and pointless end, in spite of how hard he worked to build a future for himself. Made worse when the last shot of Chris is just him fading away...Then you remember how River Phoenix died and it adds a terrible layer of heartbreak.
The mention of how Vern and Teddy both eventually became just another two faces in the halls. A sad reminder that sometimes, no matter how close you are or what sort of bonding experiences you've shared, once-dear friends may still someday drift out of your life and go their separate ways without you.
"Friends come in and out of your life like busboys in a restaurant, did you ever notice that?" Sad, but true.
The final line of Gordie's narration: "I never had any friends later on like the ones I had when I was twelve. Jesus, does anyone?"
In the original story, it's even sadder; Vern and Teddy eventually die senseless deaths (Vern in a house fire caused by a cigarette, Teddy in a drunk driving accident), Chris, as in the movie, is stabbed to death. But it goes even further when Gordie mentions how he's been getting terrible headaches as of late, the implication being that he's soon to suffer from a fatal illness, if you choose to interpret it that way.
The scenes with Denny where we see the happy kid Gordie used to be. The two siblings aren't especially close in the original story, but the death of his big brother is so much more painful in the movie, because here Gordie loses the only member of his family that genuinely cared about him.
The song itself is enough for some. Especially when you realise just how relevant the lyrics are to the story...especially with the video.
The scene after Teddy has to be pulled away from killing Milo (who is an adult and twice his size), and has a breakdown. And Gordie asking, in the voiceover, how Teddy could care so much about the father who almost killed him. All four of the boys are so sad and vulnerable in this scene.
YMMV on this, but: Adult Gordie's entire passage in the novella about his yearning to go back to the woods to search for Ray Brower's missing blueberry pot.