This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Hell Teacher Nube
Hell Teacher Nube has the episode with Hiroshi's Missing Mom being reincarnated in a kindergarten girl, Reiko-chan. The last part of the episode is pure tearjerking of the highest quality in the whole series: Hiroshi rejects his "mom" and runs away since she's treating him like a baby despite being in the body of Reiko-chan, who is at most five years old. His girlfriend Kyouko finds the runaway guy and convinces him to return... yet they find out that Nube had to exorcise Mrs. Tachida's soul from her reincarnation as Reiko-chan, so the child wouldn't be unfairly burdened with memories of her past life. Hiroshi falls to his knees and breaks down in tears, calling out to his mother and telling her that he never meant to be cruel to her and that he misses her so much — then, Reiko-chan channels Mrs. Tachida's soul one last time, so she can properly say goodbye to Hiroshi and let him know she'll be okay in Heaven. When it's all resolved, Reiko-chan's mom comes for her daughter and they go home... and Reiko-chan (who's supposed to not have any memories of this) swears she'll find herself a husband just like Hiroshi, who hears this and smiles.
Actually, we were given a massive one already in the second episode. Nube, Hiroshi and Kyouko are trying to deal with an apparently evil "Toilet Hanako" ghost, and a supernatural spirit that might be her slides around and goes through Kyouko for some seconds, which drives her to a crying Heroic B.S.O.D. as she can sense its pain... When they find the ghost, it turns out it's not the evil "Toilet Hanako"... but the spirit of a little girl named Hanako who perished in the Tokyo bombings of World War II, trapped in an abandoned house whose spot would later be used to build the school. This drives the kids to tears, but Nube calms them down and uses his cursed hand as well as his sutra to calm Hanako-chan down, then send her to Heaven in peace.
There's also Ai Shinozaki's Motive Rant, in which she explains Nube and Makoto how come she was possessed by an evil spirit that caused eyes to spread all over her body. It's because she is so lonely and drepressed over being the local Lonelyrich Kid that she developed a shoplifting habit, and the monster chose her as her host due to that. And for worse, she was about to become fully possessed by the monster, and Nube was pressuring her into telling him the reason why without telling her... because the only was to really be freed from that specific monster is to reveal your deepest secrets, the ones that attract it. Thank God everything became better.
Then there's the Whole Episode Flashback that explains how Minako-sensei died to protect a child Nube from Bakki. Which again brings the sads when Tamamo, Hiroshi and Kyouko Time Travel to help a Nube who's in danger of lose control of his Demon Hand and witness how said Demon Hand was created — and how Nube found out exactlywhat had happened to Minako-sensei's soul, and then had to fuse her soul with Bakki to both release a possessed little girl and keep Bakki sealed in his hand. The discovery quite shocks Tamamo — and makes both Kyoko and Hiroshi cry buckets.
One episode is a Pyrrhic Victory as Nube can only apologize as he seals away a set of delinquents who do not want to be returned to normal as it is the only way he can prevent them from harming others.