This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Avengers Arena
Juston dying unceremoniously after everything he went through.
His family thinks that LMD back at home was actually him and, with the ending, Hank Pym will have to go and tell them what happened for real.
Nico's final moments as she laid bleeding out in the snow, thinking that being alone was why she was afraid of death the most and muttering "Help" to the Staff of One. If you were a Runaways fan it hurt all the more.
Nara's death for the sake of Cullen, so that her lover wouldn't be forced to kill him.
Deathlocket was forced to kill Tim in order to save everyone else.