This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Kiddy Grade
Éclair singing a lullaby to the little child
Éclair, in a flashback, banging on the door of a jail cell until her fists are bleeding, screaming to be let out
When Éclair, shown in a flashback, is bandaged, very depressed, and even somewhat suicidal, yet Lumière continues to stand behind her emotionally and say that people like them cannot die
Éclair, just having waken from amnesia, first interacts with Lumière, and questions who she (Éclair) is and even tries to punch Lumière in frustration
The photos on Éclair's wall, after you realize she keeps them because she doesn't want to forget anymore
A scene in episode 20 where Lumière is dying, and even Eclipse admits that there's nothing she can do to help, and Éclair flashbacks her memories of Lumière, and says things like "I'm a G-class ES member and I...feel so helpless...!"
The big reveal at the end where Chevalier reveals that all he wanted to do was to protect his adoptive mother, and then there are flashbacks to several Tear Jerker moments of Éclair raising Chevalier, along with more of Éclair's lullaby theme
Near the end, where Chevalier asks Éclair "Can you smile for me, mother?", and then she forces out a smile so that he can die happy.
Don't forget How Dvergr spends the last few moments of her daughter Alv's life as the latter cries out for her mother...
The lyrics of both the opening and ending themes, once you figure out the bigger spoilers in the setting.
And heck, the ending theme as a whole. Once the lyrics make sense, the song it self becomes so beautiful, meaningful, touching, and overwhelmingly sad...