This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Space Runaway Ideon
The final movie. Especially if you have seen the entire series before. Even before everyone dies in exceedingly cruel ways, the movie makes it clear that there is no hope to bring this war to a peaceful end.
Even worse, both sides admitted that the herculean misunderstanding which spans the whole series should have been avoided if they are willing to talk to each other. But in the end, there is no other way to properly bring peace peace for both sides until the Ide wiped out all of them.
Even before you watch Be Invoked, the entirety of the story downfalls quick especially in later episodes, such as Moera's, Lin's and Gije's death later in the series (which affects a lot of the characters, especially Sheryl), and how the Solo Ship denizens are increasingly hopeless in their desperate attempts to escape the Buff Clan's universe-spanning chase.