This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / West Side Story
The death at the end combines the music of "Somewhere, there's a place for us, somehow, someday, somewhere," and Maria's rhapsodic affirmation of her love: "I have a love, and it's all that I have."
"One Hand, One Heart"
''Make of our lives one life Day after day, one life. Now it begins, now we start One hand, one heart, Even death won't part us now.
"I didn't believe hard enough."
In addition: "Yes... We can... We will..."
"Te adoro, Anton."
Word of God reveals that they originally planned to have Maria commit suicide at the end of the musical, like Juliet. Thankfully the final version averts this.
At the ending, the Jets all go to lift up Tony and carry him, but stumble under his weight. So a few Sharks run over to help without even thinking about it. They pause, staring at one another and contemplating, then the two groups all exit together, carrying Tony.
"DON'T YOU TOUCH HIM!!" The way Maria just shout/cries that line is devastating.
And Shrank finally displaying a shred of decency and respect by stepping back.
A meta example when you find out that Rita Moreno, who played Anita in the movie, was raped as a child, and while filming the Near-Rape Experience scene between Anita and the Jets, she had a panic attack and began crying because it triggered the memories. It turns into a bittersweet Crowning Moment of Heartwarming, however, when you find out that the actors playing the Jets all immediately stopped the scene and comforted her.
A bit of Mood Whiplash Maria having found out from Chino about her brother and her lover, runs to her room feverishly praying to Santa Maria, to make it not be true, when Tony badly beaten climbs through her window, to which a furious Maria runs at him, pounding his chest and screaming "KILLER, KILLER, KILLER!" before collapsing in his arms. The two make up and agree to run away to a place where they can be together, which in the stage version leads to an extended fantasy ballet in which they imagine both the Sharks and the Jets living in harmony and peace together, as, usually, a child soloistsings the beautiful "Somewhere". In the film the ballet is cut and the song becomes a duet for Tony and Maria, but the Heartwarming and Tear Jerking effect is still there.
Graziella wailing over Riff's death in the film. We haven't seen much of their relationship, but her reaction is enough to reveal that she loved him very much.
The final scene. After Tony's body is carried out, the rest of the Jets and Sharks just start walking away, very saddened by what's happened, but unable to do anything.
Tears of Joy for "Tonight." Even though it ends very, very badly, Tony and Maria are so in love, and so sincere in every romantic, fanciful thing they say to and about each other, it's impossible not to be moved by it.