This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Xiaolin Showdown
Omi in "In The Flesh", coming to the realization that Raimundo has never had a SECRET ELABORATE PLAN.
Omi: Raimundo really has betrayed us...
Omi's Face–Heel Turn in the end of "Judging Omi", and his speech to the other warriors before turning to Chase Young:
Omi: When I tried to lead, none of you would follow; when I tried to talk, none of you would listen. Only one person stood by me, believed in me, and trusted me. That is where I choose to go.
"Saving Omi", Raimundo is taking a savage beating from evil!Omi that results in him being pinned down and repeatedly slammed with water attacks. Raimundo actually starts crying, partially out of physical pain and partially out of despair that his friend has turned on him so completely with seemingly no hesitation or remorse.
"Return of Pandabubba" when Kimiko's zombiefied dad shuts her down when she tries to talk to him about Pandabubba, telling her harshly to get out and making her cry. Omi's assures afterwards that he doesn't believe her father is really evil and there is something else going on.
"Deep Freeze" when Dude-bot, Jack's most recent creation which included giving a robot emotions and feelings using the Heart of Jong, choosing to kill itself after being attacked by the monks while offering pleas of friendship and deciding it did not like "being human" because of the sadness it was feeling. It even starts shooting water out of its eyes for added effect.
"Omi Town", poor Omi.
Omi's disappointment in Jack at the end of the episode "The Apprentice".
During the finale when Good!Chase Young willingly drinks the Lao Meng Long Soup to give the others a chance to escape.
Also during the finale two-parter, Omi having to watch his aged friends die one after another briefly leaves him shocked, such that Dojo has to activate the Sands of Time to save him. Then Omi breaks down crying at what he's just witnessed.
Poor Omi is a magnet for these. In "Crystal Glasses" he thinks that he's going to grow up to be a villain and destroy the temple. Vlad convinces him the only thing to do is take the Long Walk- out into the cold, and never return. Before leaving, Omi looks at a picture of him and the other Chosen Ones.
Omi ends up with another in "The Apprentice" in his showdown with Jack. Jack seemingly made a Heel–Face Turn, just to predictably turn on the monks. Omi gets some payback in their showdown Truth or Lies where he asks Jack if he betrayed the monks because he was afraid he would fail at good like he failed at evil, and Omi proves on the money. Jack in turn asks Omi if he truly believed that Jack had turned good, and Omi reveals that yes, he actually did believe that Jack had turned over a new leaf. Jack is shocked, and then claims he was always planning to betray the monks, which was a lie. So Omi was right, Jack truly did try to turn good, and then gave up on it because he was afraid he would be a failure.