This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Star Trek Online
'Cloaked Intentions', the Featured Episode series focusing on the Romulans and Remans, has a couple of doozies.
In 'Mine Enemy', a Romulan miner talks about her life and the destruction of Romulus:
"My family and I have been here all of our lives. It's a hard life, but it is all I know.
My mother used to tell me tales of the homeworld. She'd talk about the plazas in Rateg, or vacations she spent with her father on the shores of the Apnex Sea. And she would tell me about my father, who gave up his place in the military evacuation ship so she could live. He ... he never knew he had a child.
She would tell me these stories, and later, when she thought I was asleep, she would cry."
In 'Frozen', the next mission in the story arc,we get to hear from the regular grunts of the Reman resistance... and it's not pretty. An example:
Player: Why are you fighting the Romulans?
Reman: My wife was working on a mining ship. This was not a military vessel. It had no weapons and limited shields. It was not in restricted space, and it threatened no one. The Tal Shiar captured it and "questioned" everyone on the crew for weeks. My wife died screaming in a Tal Shiar interrogation chamber. I fight for her.
Player: What do you want when this conflict ends?
Reman: What do I want? I cannot answer that question. All I wanted — all I will ever want — died with my wife.
The Cardassian mission 'Badlands', when you learn the fate of the parents of Joshua Riker.
In the mission "What Lies Beneath": When you find the latest victim of the crazy murderer, only seconds after she was begging for you to help her. AND once you realize the murderer was a hologram whose program degraded due to the radiation down there.
Most of the Romulan Captain's story arc. It starts off with your colony being invaded and it gets worse from there. The penultimate mission shows that just about everyone abducted was or is in the process of being turned into more Elachi.
The destruction of the old Earth Spacedock. The old one had been there for three years and to see it pulverized by the Undine...
In Delta Rising Gaul slaughtering the Talaxians, all in an attempt to anger you. Neelix doesn't rise to the bait and is just heartbroken.
In the mission "Dust to Dust" the revived and turning-into-a-Kobali Harry Kim, frightened and confused, doing everything he can to get back to Voyager, not realizing what he's doing.
The deaths of the Preservers in "Blood of Ancients". They spent millions of years in stasis in the hopes of one day meeting their descendents, only to be dispassionately wiped out by the Iconians.
For the ground portion of Broken Circle, you will quickly round up a platoon of Red Shirts as backup, making the early parts of the mission quite easy. But as you travel between repeated engagements with M'tara expect your forces to dwindle until it's just your usual away team again. After that's done, the next map consists of flying through the ruins of your fleet which had been so huge at the start of the mission.. Your listed objectives are to scan for survivors, but the game cuts you off with instructions to just leave when you try.
In the same mission there is a point where a Herald dreadnought is too powerful and Tom Paris gives the order to fall back. A Klingon captain says "I can't do that.. Today is a good day to die", as he revs up his engines and does a kamikaze to give everyone else a chance at hitting it.
"Butterfly" has the team decide to use the Krenim weapon to stop the U.S.S. Yamato from discovering Iconia. It would stop the Iconian War altogether and bring back Romulus. Then, to your horror, you discover that, by doing so, the Borg have taken over Romulus.
"Midnight" has you going back to the day the Iconians were eradicated. Just like Jean-Luc Picard postulated, they did nothing wrong. They might have been arrogant, but they were trying to protect other races and those races didn't like it.
And Sela's reaction when she realizes the destruction of Romulus, and probably the entire Iconian war, are all her fault. It's the only moment in the game where she truly loses faith in herself, and for a second you think she might actually shoot herself out of despair.
The last two TOS missions before the jump to the 25th Century is this:
In "Painful Omens" you get to see what caused the Tholians to go after the Na'khul - time travelling Na'khul massacred a colony of Tholians and their Queen opted for a Taking You with Me to gain revenge, but not before telling the other Tholians that the Federation was not to blame.
In "Battle of Caleb IV" you perform a Last Stand against a fleet of Klingon ships, giving your former Captain and the other ships a chance to escape. Even more, the captain, now Admiral, parts with a touching "Godspeed... and give them hell!"
"Survivor" has the player, Agent Daniels and Sela arrive at an abandoned Tal Shiar prison and find evidence that Tasha Yar and the crew of the Enterprise-C is still alive when you hunt down the T'Nae from "Temporal Ambassador". You're given Hope Spot after Hope Spot that, somehow, Tasha is still alive only to learn that Tasha was one of the last to die, passing of old age and that her last request to T'Nae was for her to tell everyone the story of the Enterprise. At the end, Sela, once again in jail, is confronted by a Starfleet officer, who gives her a hologram of Tasha. Sela ends the mission by asking the person "Tell me about my mother...", the same thing Guinian asked Data of in "Yesterday's Enterprise".