This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Scott Pilgrim vs. The World
Ramona calling Scott "Another Evil Ex waiting to happen."
She manages to drive the point home even harder with four simple words: "Here's your damn list."
The part where Scott breaks up with Knives and then the song Teenage Dream (by T-Rex, NOT Katy Perry) starts playing as he's on the bus was also really sad too.
Also, the part where Scott sees Knives in the crowd after defeating the Twins.
Beautifully accompanied by Broken Social Scene's lilting, wistful Anthems for a Seventeen Year Old Girl.
The whole part after the final fight up to the credits. Knives and Scott seems to finally kick in and it looks like they're gonna try again (Knives even commenting that his hair has grown longer, and cue Scott with his hat running gag. She just dismisses it and tells him he should cut it, to which he agrees). Ramona turns around and starts walking away, and Scott and her says their "final" goodbyes. Knives, however, doesn't want Scott to suffer and stay that way, so she tells him to get her back. Cue Scott and Ramona, walking towards a subspace door, ending the scene in a sunset. The REAL Tear Jerker, however? Scott's Ramona cute song, in a beautiful and sad reprise, until the credits, and the "CONTINUE?", show ups.
The countdown just before the credits can make one's heart ache for some reason.
Speaking of endings, the alternative ending, where Ramona leaves town tearfully, is really sad.
Knives's little "Oh" after Scott breaks up with her. She slowly looks down at her own heart right after that - you can almost see it appearing in cartoon form and breaking in two, even if the movie itself doesn't go that far.