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Tear Jerker: Death Battle
In general, any episode where your preferred character dies will be this to at least a few people.
Seeing the Teenage Mutant Ninja Turtles viciously murdering one another in cold blood is heartbreaking. The lingering pause on Leonardo's face is not the look of a winner; it is the face of horrible realization.
Goku vs Superman
Many whose childhoods went to Goku becomes this somewhere. Especially heartbreaking is the expression on his face and his Death Cry Echo.
Their battle ended up completely destroying the planet Earth. Doubly so for Superman as he ended up destroying what he was sent to protect. His expression and the utter silence at the end of it all really hammers it home.
What's worse is just how petty it is. Superman never wanted the fight at all and Goku just wanted a good fight (and to stop what he thought was a possible alien threat), but unfortunately he and Goku just kept getting pushed to the point where they couldn't hold back anymore.
Mike Haggar's defeat at the hands of Zangief was this for Boomstick In-Universe. Possibly for Wizard as well, though he focuses on giving the post-battle explanation as he usually does and only joins Boomstick in mourning when asked by a fan if his preferred contestant ever ended up losing as a result of their research.
Felicia's defeat means that the orphans she cares for just lost their caretaker.
Really the case for any character who has a family/True Companions/In-universe Fanbase that ends up dying. Who knows how they'll react to the news to say nothing of Leonardo and his brothers...
Fox McCloud vs. Bucky O'Hare is probably one of the most depressing match-ups to date. Fox loses one of his closest friends, Wiz and Boomstick see one of their favorite characters get blasted to smithereens, and Bucky loses just about everything. It's hard to say anyone actually won that fight.
Luigi vs. Tails is possibly even more depressing. Although the episode has plenty of Funny Moments, it doesn't change the fact that it's a battle to the death between gaming's biggest Butt Monkeywho lost and an innocent kid no older than 11 who won.
The expression of sheer terror on Venusaur's face as it burns up in Charizard's flames.
The Pokemon were leveled up by rare candy instead of being already fully grown. Three kids just fought to the death.