This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Science Ninja Team Gatchaman
Joe's dying speech to the team in the first series finale of Gatchaman.
And Kentarou Washio aka Red Impulse's death. Made even worse some episodes later when Ken is tricked into believing he may be alive, but it was just a trap from the enemy. Seeing Ken watch the sunset in tears and being unable to finish his usual afterthought speech is just cruel.
To add to that, Ken is tricked into believing his father is alive once more in an episode of Gatchaman II, but it again turns out to be a trap by the enemy. At the end, a melancholy image song plays as Ken muses over the events.
How about the end of episode 3? Ken befriends a boy in a similar situation to his, but unlike Ken this boy's father is dead and Berg Katse has taken the man's identity. Ken fights him, drives him off and lets the boy hate him for making his "papa" go away again because he knows the boy's pain all too well. He just doesn't have the heart to crush his dreams even when they've already been crushed, and the sad music accompanying a more melancholy "hang in there, Gatchaman" doesn't help.
Ryu quitting the Science Ninja Team after his mistake of oversleeping gets the team in trouble. Once again we hear the sad music as he sits alone on the train, mentally apologizing to the team. "I'm sorry, guys, but I just can't let this go". This troper gets teary-eyed just remembering that train scene.
The ending of the Star-Crossed Lovers episode. A happily engaged couple is captured by Galactor and made into cyborgs, and by the time the Science Ninja Team arrives to save them it's too late; they've been fully corrupted and when Ken tries to talk to them they're already ready to go through with ending their lives. They share one last tearful kiss and embrace before their cyborg bodies explode, and the team is left to deal with the aftermath while sad music plays in the background. The fact that the couple resembles Ken and Jun doesn't make things any easier.
The ADV dub even adds a robotic echo to the two lovers' voices after their cyborg modifications to drive home the fact that they've been permanently altered with no hope of saving them.