This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Mobile Suit Gundam 00
The whole point that kick-started the entire plot. On one hand we see great countries continue to thrive marvelously even after fossil fuel became scarce, but then there's the utter hellhole like Krugis, Setsuna's birthplace, so wrecked by war and now assimilated into Azadistan... Given that the show ran in 2007 when The War of Terror was still strong in a lot of people's memories, it makes the series even more tragic.
Pictured on the right: Although Louise was a very unpopular character, the scene where Saji visited her in hospital and gave her the ring that he had been saving up to buy for her, and she started crying and saying she couldn't wear it, and revealed that she'd lost her left hand in the attack that killed her family.
And she ended up lending support to A-Law, becoming one of it's soldier all for the sake of revenge against Celestial Being in season 2. Imagine how shocking it is when the Trans-Am Raiser is activated for the first time and reveals that she and her lover Saji is fighting on the opposite side...
Episode 23 ends extremely tragically when Lockon Stratos, the series' resident badass is killed in battle, sending the remnants of his Gundam and his Haro, which was pleading for Lockon's life the entire time, back to the main ship. Who'd ever think that a constant Say My Name could be so depressing? It's all made even worse when you consider that Lockon had survived a perilous situation within the previous two episodes (albeit with eye damage); no one expected Sunrise to let him survive that, only to kill him off so soon afterwards.
In episode 24 Haro and a letter for Lockon are placed in Dynames with the orders to 'go where Lockon is, so he's not alone'.
The 00 writers outdid themselves in episode 24, with Lichtendahl attempting a Heroic Sacrifice to save Christina... but it doesn't work, since not only is she fatally injured as well, but the Ptolemy bridge explodes right after that, completely erasing any chance of survival. Honestly, that episode becomes a sobfest just four minutes in, what with Haro going "Feldt, I'm sorry; Feldt, I'm sorry," and a grieving Feldt tearfully saying that it needn't apologise because what happened to Lockon wasn't its fault.
Tieria, seemingly near death, ejecting the GN drive from the destroyed Nadleeh and saying that he can finally reunite with Lockon during episode 25.
Imagine being in Lyle Dylandy's place. Lyle watches his family taken away by world's conflict, and later ended up hired by an organization that, in a sense, killed his only brother as well. Feldt's advance on him seems less like a typical relationship problem and more of an insult on personal level.
In episode 7 of season two, the scene where Soma (or more accurately, Marie) and Sergei part ways had quite a few fans sobbing - particularly when Marie tells Sergei that her Soma personality had wanted to be his daughter.
The intro to episode 4 of season two. The flashback shows the first time Allelujah finds Marie as a child as well as the horrible conditions the two had to put up with in the facility. If that wasn't a Tear Jerker already, we then get to see how Allelujah got his name in the first place.
Allelujah: That was the moment when she baptized me... in the truest sense of the word.
Andrei's breakdown when Soma forces him to face that he should have tried to understand his father is extremely sad as well.
Anew Returner's death and final conversation with Lyle in episode 20 of season two, as well as Lyle's resulting Heroic B.S.O.D.. What really drivse the nail on this one is the fact that Trust You immediately kicks in right after Anew's last words...
Nena's fate throughout the series can be seen as disturbing, to say the least. She blew up the wedding because she's annoyed by seeing people happy when she's tired from "work" (shooting at soldiers, munition workers and the like). Eventually, it was revealed that, instead of being the people that can change the world, their actual "work" was to be shot at as part of Ribbons's plan to accumulate hate against Celestial Being, and allowing for him to deploy the GN-Xs and the Innovades. Because the Trinities had little support from the crew of the Ptolemy and used interior machines, if one of them was killed, they wouldn't be able to fend for themselves as their tactics relied heavily on co-op skills. With some luck, they survived several contacts with the GN-Xs, so Ribbons had to dispose of them through both Ali and by taking advantage of Louise's mental instability and thirst for revenge. She's pretty much an ill-treated Woobie, Destroyer of Worlds.
Special mention goes to Nena, whose psychopathic tendencies result from her spending her first 10 years in stasis. Thus, despite being physically 17 years old, she possesses the mentality of a little girl. While it might not justify some of her actions, it would account for why she acts in such a fashion.
In the movie, Andrei's Heroic Sacrifice as he self destructs his suit while slashing a ELS ship to pieces, keeping it from reaching Earth, all while recalling the similar sacrifices his parents made. It doesn't help that the music starts picking up at that very moment.
"I am a soldier of the Federation forces, serving to protect the people!"
Graham: Young man! Do you need a beacon to the future? I, Graham Aker will gladly oblige! This is not death... (*activates the Brave's self-destruct sequence) IT'S FOR HUMANKIND TO LIVE ON!
Allelujah being forced by his Split Personality to destroy a Super Soldier facility filled with children in episode 11. The kids would've lived miserable lives as HRL soldiers similar to him, but one can't help but feel sorry for the poor guy helplessly screaming as Hallelujah forced him to pull the trigger. And then he asks Sumeragi for a drink.
Kinue's death at the hands of Ali, followed by poor Saji being called in to identify his sister's body (which happens before the opening credits begin!) or him speaking with Kinue's boss, who feels guilty because he didn't do anything to stop her from pursuing such dangerous work. And as a result, the last person imaginable to be anywhere involved in Celestial Being's war still ended up getting roped into it.