This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Fallen King
Almost all of Fallen King is a tearjerker when it's not being awesome. Sometimes it's both.
Isis and her companion realizing that Yugi is destined to lose the duel.
Yami Yugi's reaction when Yugi is overcome by the Shadow Realm.
This part as everyone waits for the duel's conclusion.
"Do you remember when you first met Yugi?" Téa whirled on him violently with her intense blue eyes piercing into him. Sudden hatred swirled into her. "Don't you dare talk like that!" she screamed at him. "Don't you dare talk like he's gone! As if this is good-bye! As if...As if..." Téa wrapped her arms around herself in effort to calm her emotions. "As if he's already lost." Her blue eyes brimmed with threatening tears. She hated crying.
Everyone's stunned reactions when they realize Yugi lost, and what it means for them and the world.
Yami Yugi agreeing to Joey's demand to leave, realizing that he never truly had their friendship.
Any time Pegasus or Yami Bakura taunts the heroes about Yugi.
Joey's decision to break the Millennium Puzzle.
"It all started with this Puzzle," Joey whispered. "Yugi made a wish on it... And now only evil is coming from the good that surrounded it..." He smothered a sob. "Y-Yugi, I told you... Our friendship is something special and I promised to be your f-friend f-forever..." Joey raised the Puzzle in the air and smashed it on the stone ground. The Puzzle broke instantly, scattering the mystical gold pieces everywhere.
Joey and Tristan being forced to battle Téa, who controls Yugi's Dark Magician.
Joey throwing Ryou Bakura off of the helicopter to his death.
"I'm protecting my friends," he replied. Though his voice was steady, his body trembled fiercely. Things had certainly changed. "He's not our friend."
Joey realizing that Serenity will be blind forever.
"Serenity," he whispered. Another broken promise to add to his list. But this one had to hurt the most. Yugi was not only dueling Pegasus to save the lives of his Grandpa and the Kaiba brothers, but for him, for Joey, for Serenity's operation. So this is what it feels like when all hope is gone? Then that was that. Serenity was destined to be blind after all.
The ending of the story. To save the world, Joey, Tristan, and Téa cannot be friends.
"We can't be friends," the blonde repeated. "We're a danger to each other and to everyone else." "What are you talking about?" demanded Tristan harshly. He was tired of being jerked around. "How can we hurt each other? We're not under anybody's control." "That's just it. We could be." [...] "They used our friendship as a weapon," interrupted Joey. "They'll do it again, and they're getting stronger. We all hold something that Pegasus wants. And if they get it, then there will be no stopping them. If we separate, then at least that's something for the world to hold on to, even if..." Joey gave a grim look. He couldn't even finish his sentence. It was too much. If we fail, then there's maybe a chance for someone else to do something about it... Before the world ends.