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...
Presumably, Luigi goes after Tails because of Mario's death at the hands of Sonic. All he accomplishes is also getting killed and making a child a killer. On top of that, Peach survived her battle and must live without the protection or kindness of the brothers. Another layer there: Link lost Zelda in that fight. Yeesh.
Zelda and Link reincarnate. She'll be back. We should be more worried about Link, now armed with whatever Materia he looted from Cloud's corpse, stomping over to the Mushroom Kingdom and decapitating Peach. It leaves her kingdom leaderless and the Mushroom World short three of its seven Star Children.
It'll be worse if Bowser's abducted Peach when Link shows up. Why? Because then Link will hunt Bowser down to get to Peach. Bowser has eight kids, all of which will either be orphaned or slaughtered as collateral damage in the hunt.
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 worst part is that this all happened right after the Year of Luigi. Right after Luigi finally gets his time in the limelight, he gets defeated in a Death Battle.
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.
Godzilla, mad, rampaging monster and destroyer of Tokyo who generally only ever saves the world by accident on the rare occasions where he does, utterly destroys Gamera, the defender of earth and friend to all children. For only the second time in Death Battle history, what is essentially a bad guy was pitted against an unambiguous hero. And this time, the bad guy won. As if that weren't bad enough, the death toll absolutely had to be enormous, and to top it all off, if the battle universe held true to the Gamera series at all, there were probably small children watching as their hero and earth's only hope was completely obliterated. GOOD. GOD.
The final scene of Tigerzord vs. Gundam Epyon is actually cartoonishly hilarious, but as always, thinking about this battle too hard is...troubling. Zechs/Milliardo has serious emotional issues and Zordon just watched one of his peacekeepers explode along with 3 of their strongest weapons. Nothing good can come from Noin and Tommy's deaths, knowing this.
Kirby is quite clearly devastated after his Star Rod is destroyed. It's even worse after Buu launches his Planet Buster and Kirby looks up from crying with tears still in his eyes. He knows that he had to dig deep in a VERY short period of time to counter this. If one of the most powerful weapons in his canon and last moves, his Hypernova form, is thwarted by Buu, then his entire home will be vaporized in return.