This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / House of Anubis
Victor's devastation when he learns that Vera works for Rufus.
His reaction to learning Sarah died. Sure, he's a villain, but after all those years, he finally found his friend again, just for her to die the next day.
When he receives the ring from Nina, the ring his father left for him. After two seasons of trying his hardest to live up to his father, become immortal for him, finally make him proud, it is very hard not to be moved when he starts tearing up.
Poppy's speech during her father's hearing.
Fabian's reaction to Nina disappearing in the Sennet game thanks to his wrong move. It is utterly heartbreaking, and grows worse by the minute, culminating in him finally breaking down and running to find Mr. Sweet.
Then there's some Mood Whiplash when he and Amber find out where Nina is, and he suddenly breaks out into a goofy grin and laughter of joy- which causes some tears of joy on its own.
Fabian's reaction to learning Nina will not come back. He has the same reaction as he does after Sennet, only this time, he knows he will not see her again.
Later, when Amber reads Nina's letter out loud for him because he can't handle reading it alone.
Jerome breaking up with Joy on stage. Heartwrenching, because he only did it because she was going to do the same with him as a way of humiliating him. Thing is, they had both fallen deeply for each other, which turned this into a very painful scene to watch.
Similarly, Joy's heartbreak when Fabian rejects her in the harshest way that has happened in the show. She was so devastated by what happened that she ended up leaving Sibuna because she couldn't handle being around Fabian.
Small, but look at Patricia's reaction to Joy's disappearance- besides her hostility to Nina and her determination to find her friend, there was also a ton of desperation, and it gets a little worse when she and everyone else believe she's going crazy. Yes, she did a bunch of bad things, but it's hard not to feel bad when she was suffering these breakdowns.
Amber breaking down during House of Trickery/House of Recap. To elaborate, the had spent the entire night locked in the tank room in the gatehouse all alone, only to be found by Victor and Miss Denby. Victor then forced her to lie to her father about what happened, and after her father reprimanded her, she was forced to apologize to Victor. Then she was blackmailed into betraying her friends. All this, in one episode.
How Fabian reacts to Jasper's betrayal. He keeps trying to rationalize and believe that Jasper is not a bad guy, but finally is forced to face facts when Jasper lies right to his face.
Similarly, and probably even more tear-worthy, is how Eddie takes the news of learning his father is on Team Evil. He spent almost the entire season trying to convince himself that his father would not do the bad things that his father does, indeed, end up doing, and his father casually betrays him constantly without any sign of remorse. It gets worse when Mr. Sweet becomes a sinner...
KT getting ostracized when Sinner-Patricia managed to trick Sibuna into thinking the sinner was actually KT. Eddie went so far as to steal KT's key, the one her dead grandfather gave her.
Amber choosing Fabian and Nina as prom king and queen over herself and Alfie at the end of series one.