The mines underneath the Empire's tower are running out of oxygen and Kotaro is trying to figure out how to kill Raphaello. Marcello breaks down crying because he can't even look at the sun before he dies. Kotaro does what he can and writes out the formula for sunlight, before he starts crying too because that's how little he can do to save a life despite being a scientist.
Alternatively, in the anime Amigo's You Shall Not Pass!, strapping himself to the door just to keep Kotaro safe also qualifies.
The death of Kaos and his BT, made more heartwrenching when Kaos can finally die happy knowing Teppei will surely triumph.
B'T X Neo ends on a very bittersweet note, where Teppei tears open the capsule containing the piece of the sun, releasing such power that it engulfs each of the Four Spirit Generals along with Teppei. Each of them share parting words with their B'Ts as they understand that to stop Raphaello, they must give up their own lives. Kotaro shares their fate as the Area goes up in a blaze of glory. This is framed around Karin in the future telling their tragic end to children of a new Machine Empire. That the heroes we followed passed on stings, but knowing that their efforts were not in vain and that their legend lives on would even make X tear up. The final shot of a resting Teppei and X is the perfect note to end it on.
The manga has the final fight take place in space, and as it seems everyone won, and the Emperor was finally defeated... They realize they don't have the power to come back to the Earth. The Four Spirit Generals and X end up sacrificing all their energy so that X can achieve atmospheric reentry with Teppei and Kotaro in tow, making this less of a downer ending as Teppei fondly reminisces his time with X, and prepares to get back to his old life with Kotaro.
