This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Power Rangers RPM
The state of planet Earth during the series. Humanity is driven into near extinction, huddling in one city to survive. The world outside of Cornith isn't very hospitable. There is constant fear of being attacked by Venjix army and knowing that if Cornith falls, humanity is pretty much doomed at this point. While the Power Rangers did eventually beat Venjix who knows how long it would take before Earth can fully recover and the physical, psychological and emotional scars of Venjix invasion and rule won't heal that easily.
From "Ranger Green": Ziggy when he discovers the truck he was driving was carrying medical supplies. At one point he has a look on his face like he knows he's already a dead man.
Linkara points out that Ziggy successfully escaped the city and the cartels only to try and get back in because there was literally no where else to go!
From "Ranger Red": Scott watches helplessly as his brother is shot down in front of him. Marcus's last words: "SCOTT!"
From "Ranger Yellow, Part 2": Andrews sacrificing his life for Summer.
From "Doctor K": The entire episode, especially Dr. K's flashbacks to her life growing up at Alphabet Soup, counts as one giant tearjerker. Someone please give her a hug.
Government Agent: Happy Birthday! Make a wish!
Doctor K: I wish I could remember my name...
From "Ghosts": Dr. K's reaction to seeing Gem and Gemma for the first time since Venjix attacked Alphabet Soup. And she smiles, even with tears running down her face.
From "Ancient History": Dr. K's exchange with Colonel Truman in Corinth HQ.
Dr. K: We all do things in life we regret. But if we don't learn from them, and move towards a brighter future, we pay twice.
Colonel Truman: I've already paid more than most.
Dr. K: I know. And if I could access the space-time continuum long enough to undo what I did, I would. That's not possible.
The deletion of Gem and Gemma is one of the saddest moments in the franchise, right on par with the deaths of Zordon and Kendrix.
The last few scenes of the series has two. One where they were returning their morphers and Ziggy quietly, and not just a little bit sadly, says "Get in gear". And the second where Dillon returns to a flower we saw in the first scene.