This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Romeo X Juliet
When Juliet and Romeo meet and fall in love. Because in this version they're actually likeable characters, and the knowledge of what's to come just kills you.
Episode 11, the "Romeo and Juliet are officially a couple" episode, consisting of mainly the two leaving the city for a day to just wander around all lovey-dovey. Like the last example: nothing obviously tragic, until you remember what you're watching.
And in the same episode, when Romeo lets Cielo go to live with his new mate.
Episode 6: Dr. Lancelot'sHeroic Sacrifice, and Juliet's anguished reaction to it.
Juliet: I killed him. I might as well have set him ablaze.
Episode 10: Juliet, after the ambush of the Capulet base, is guilt-ridden to the point that she loses the will to live and wanders throughout Neo Verona until she collapses in front of the convent Portia, Romeo's mother, resides in. The tears continue when Portia tearfully apologizes to Juliet for Lord Montague's monstrous acts.
To cap it off, there's the anime's version of the balcony scene, which crosses with Heartwarming and Awesome on Romeo's part.
Episode 14. Petruchio. Petruchio's death in Romeo's arms. *sob*