This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Gone with the Wind
After escaping Atlanta, Scarlett and Melanie find the land around Tara ravaged, all homes except Tara in ruins, and Scarlett's mother dead of typhoid.
Take a good look at Mammy when she's telling Scarlett what happened. That is the look of a woman who has lost the woman that she's probably raised since the time she was born and nearly lost two girls that she also helped raise. It's heartbreaking to say the least.
In the movie, when they get the casualty lists from Gettysburg, there's a band playing a cheerful melody. One of the boys in the band has just found out his brother has been killed and cries while he plays.
That scene is even sadder in the book, where Scarlett is reading the names and realizing that the list includes many of her former beaux and childhood playmates. Her relief that Ashley is alive is eclipsed by her devastation. It's one of many scenes in the book that show that Scarlett isn't completely heartless.
The shot of Scarlett wandering dazed through Atlanta, after the bombardment has stopped, the camera pulling back to show dead and wounded Confederates as far as the eye can see.
Ashley's recital of the Episcopalian funeral service - from memory - over Gerald O'Hara's grave when he realizes that the Catholic service is far too brief for the assembled mourners, who were expecting a much longer and more emotional farewell. Also qualified as a Heartwarming Moment.
Scarlett's miscarriage. She's utterly devastated as this is the first time she's genuinely wanted the baby and cannot bring herself to call for Rhett, who she thinks wants nothing to do with her.
Gerald O'Hara and Bonnie, two people Scarlett loved, dying the exact same way.
In the movie, Scarlett's reaction really drives it home
Mammy telling Melly what Rhett and Scarlett have been saying to each other since Bonnie died. In particular (in the movie, at least), when she says that Rhett has been refusing to allow the funeral because, he says, he won't put Bonnie in the dark because she's afraid of it. Couple this with how much he doted on his daughter and his refusal to allow the 'leave her to scream' policy (at a time when it was common practice!), and it's bad. But then imagine, instead of Mammy's tearful words to Melanie, Rhett yelling it through the door with his voice cracking...
Bonnie's death itself is really sad as well.
A dying Melanie telling Scarlett, "Be kind to Captain Butler. He loves you so."
Melanie's death scene, and the one immediately afterward, when Scarlett finally realizes that Ashley loves his wife more than he ever will her.
In that same scene, Melanie asking Scarlett to take care of not only Ashley, "as you've looked after me for him", but also Beau. Scarlett readily promises that Melly's son shall want for nothing, while at the same time begging her to try and fight off death.
Melanie: Promise me.
Melanie: Look after my little son. I gave him to you once before, remember? The day he was born.
The ending, when Rhett rebuffs Scarlett's genuine entreaties and leaves her. No matter how much you might dislike her (and with good reason), you realize, this is a woman who within approximately one year (and even less time in the movie) has lost an unborn child (the first one she really wanted), her living child (her favorite), her best friend (and she didn't even realize this until the woman was on her deathbed), the man she thought she loved and now the man she did love. That is hard.
It becomes even more sad considering Rhett, who has loved Scarlett for years and put up with with her attitude, finally gives up on her and leaves just when she realized she reciprocated that love.
(from the book, right before he delivers the immortal line) "I wish I could care what you do or where you go, but I can't."