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Tear Jerker: Summer Wars
Sakae's death. Also, Sakae's will being read to the family, where she pleads to them to take Wabisuke back into their lives. This is, without a doubt, one of the saddest anime movies ever made.
"I want all of you to sit together and eat as a family. As you have always done. The worst things in this world are being hungry and being alone."
The scene where Natsuki starts crying, and then Kenji holds her hand as she's crying, right after Sakae's death, is one of THE saddest moments in anime history!
Brina Palencia channeled her very sad side by remembering the death of her own grandmother and that's what you hear in the dub. Yes, the crying is REAL.
The manga adaptation makes Sakae's death worse when Kenji has a nightmare where he's alone in a dark, destroyed OZ, the bodies of defeated avatars all around him (King Kazma included) and finds Sakae attempting to face down Love Machine all on her own with the same weapon she'd pointed at Wabisuke. Cue Kenji jerking awake only to realize something's wrong.
Seeing the sardonic and to this point quite unlikeable Wabisuke have a BSOD once he finds out Granny Sakae just died. After rushing back to the house (and by the look of the car, going through anything in his path), he cries out for Granny with such a look of despair on his face it breaks hearts.
In a similar vein, seeing reclusive and sharp-tongued Kazuma break down completely after Love Machine defeats him a second time and devours King Kazma. He breaks down crying not only because he lost—and to him that's kind of a big deal, since many users know him as a world-renowned fighting champion—but also because he feels that he couldn't protect his mother and unborn sister from the incoming threat.
The Casino scene, where Natsuki loses thousands of accounts due to a monetary distraction. As the computer says 'insufficent funds, do you wish to quit?' You can see a silent It's All My Fault written on her face.