This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Gundam Build Fighters
During the fight between Wing Gundam Fenice and Star Build Strike, you see a flashback of Fellini playing and battling with Fenice ever since his childhood, and Fenice's evolution from a standard gunpla with a slightly-customized paint job into its current Ace Custom form. It's enough to bring a Manly Tear to your eye. Given his age, this is likely an early High Grade Wing Gundam, the type sold in stores during the first showings of Gundam Wing in America, and is technically inferior in many ways to the modern models. That's right - the Wing Gundam Fenice started its life as the same suit countless bright-eyed Gundam fans likely bought as their first and assembled. Still remember where yours is?
Mao crying on a riverbank after losing to Sei and Reiji. Made worse by the fact that it's Mao.
Fellini's face when a berserker Aila is destroying his Fenice after Fellini had lost and couldn't do anything to stop her. It is made worse as we saw what they went through from episode 15.
Speaking of which, what happens to Aila in the same episode as above. Already upset that her next opponents would be Reiji and Sei, Aila finds herself driving to a mental meltdown after being mind-raped by the Embody System. As if that wasn't bad enough, The Stinger shows the look on her face when she sees Reiji with her cracked helmet. After having found new enjoyment and new friends, Aila is clearly terrified that she's going lose it all, including her own mind.
Gets even worse in the next episode which opens up with Reiji, not realizing that she wasn't in control of her actions, hatefully yelling and berating her. The reactions she has during this scene are practically soul-crushing.
Aila's life up to Episode 21 is nothing but this. It's revealed that Aila was an impoverished orphan who only garnered attention due to her ability to predict outcomes for Gunpla battles. Any sign of care Nine showed to her during his scouting trip disappears after she is recruited. Being seen as nothing more than a tool for Team Nemesis to win Gunpla battles, she was pressed to being a subject for the Embody System, a dangerous and illegal system that increases reaction time at the cost of consciousness and mental well-being. And it's not like she has much say in the matter, as Nine threatens to toss her out to the streets again if she refuses to follow Nemesis' orders. Thank God Sei and Reiji gave her a way out of this living hell by the end of Episode 21.
Played With in episode 22, where Julian's grandfather, in the hospital suffering from a severe heart attack, begs Julian to take his Gunpla and fight one final time with it. Turns out this was a ploy to get Julian to be interested in Gunpla again and reestablish his friendship with Yuuki. Arguably goes full-circle, back into Tear Jerker, at least in-universe, where, after Julian's and Yuuki's friendship reforms, his grandfather is seen crying with happiness.
Back to sad ones, in episode 24 It would be enough for Yuuki to be forced into the Embody System, but just having it explained to him that he is going to become just like the second Meijin, the complete antithesis of everything he believes in, and there is nothing he can do about it is just heartbreaking to watch.
Episode 25 reaches the peak for this show with Sei and Reiji's goodbye.