This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Kamen Rider OOO
#16. Gamel. Full stop. He spent an entirety of the day trying to find Mezool, and when he does, he gets absorbed by her and becomes a massive monster.
#20. Ankh spent the past two episodes trying to get his Core Medal back from Kazali, and at the end of the episode when he sees at Eiji using the TaJaDor combo to fly, he has a look of such desperate longing on his face. When he absorbs the Medals and tries to regenerate his wing only to have it break apart on him, it's the look on his face that makes you feel sorry for him. Watching him stumble away, without any biting words or rude gestures left in him, is just sad.
#40 is devoted to Ankh's Character Development and how he can't bring himself to truly hurt Eiji or Hina. Unfortunately, he gets absorbed by Lost Ankh.
#41: Mezool goes berserk and tries to eat humans alive just so she can feel genuine love. Ultimately she dies in the arms of Gamel, the only one who truly loved her—which she never noticed, because she was a Greeed.
#46. Gamel's death. After he goes on a rampage, converting everyone and everything he can find into Cell Medals in a futile attempt to bring Mezool back, he's defeated by Gotou and Date. Once he returns to his human form, the audience sees him trying to give a lollipop to Mezool's last Core Medal. Then, the last thing he sees is a hallucination of Mezool where she happily accepts a lollipop from him. Poor, poor Gamel...
What's worse, before Mezool died, he'd spent the entire day complete and not gone beserk like she did. If it weren't for her death, then he might have been fine...
Ankh's glorious Heroic Sacrifice, giving up all of his Core Medals to Eiji for one final TaJaDor combo to finally stop Dr. Maki and save the world.
Ankh's backstory, as revealed in a tie-in novel, is pretty damn tragic. Before the Greeed first came to be centuries ago, he was not yet Ankh, but rather the King of Birds, a being who felt so free and desired only to see the beauty of the world. When the Tenth Medals were removed from the sets and the Greeed were born, Ankh's senses were distorted, leaving him unable to know the world as he once did. One day, he met a young blind girl who desired nothing more than to see, so that she could know the world's beauty. Despite his hatred of humans, Ankh found himself befriending the girl, and created the Condor Yummy to help grant her wish. But when her desire became reality, she saw the horror of the method by which her wish was granted: the Condor had been gouging out the eyes of numerous random peopleuntil it found a pair that worked for the girl. She lost her mind at the sight of the carnage, and as she lay dying she asked Ankh to once again become the King of Birds, so that might protect the weak. Not long after, Ankh was betrayed by the King OOO, a man he believed to be his ally, and for all his trouble was sealed away for 800 years, leading into the first episode.