The time has come for me to finally answer this! My answer is, without a doubt, Meteor Impact!

Though it may seem like a confusing answer at first considering Midori barely makes an appearance, I assure you it's the story that paves the way for midochia's future. Meteor Impact solidified that Chiaki's story can only become complete with Midori, and Midori's story wouldn't exist without Chiaki.

The nuance for us as readers—to know why Midori has made it his purpose to peel away every layer of Chiaki's mask—wouldn't exist if we weren't aware of the significance of these walls in the first place. The reason we know what Chiaki's glasses represent, and the implications of Midori's infatuation towards Chiaki wearing his old glasses; it is only because of Meteor Impact, that we can feel the depth of their relationship.

It ties the context of every midochia story together. In Study Heroes when Midori first learns Chiaki is more than he presents himself to be. In Sweet Halloween when Midori got glimpses of a vulnerable Chiaki, panicked at the idea of showing his weak side. In Comet Show when Midori wouldn't allow Chiaki's years of hard work to wash away and let him leave the stage he knew he adored.

During Climax, when Midori showed Chiaki what it means to be a hero—with a tokusatsu quote no less—it is because of Meteor Impact we know what that meant for Chiaki, the very same Chiaki who could only shyly encourage himself with tokusatsu phrases all alone in the past. For all the time Chiaki had desperately clung to hope, when he could only endure and wish for a brighter future; that very moment of Midori thanking Chiaki for all he has ever done for him is what Chiaki was waiting for. Though Chiaki never dared to wish for a reward, it was all of Chiaki's rewards culminated.

Though Midori doesn't know the exact details of Chiaki's past, he has long ago realized the reason Chiaki clung to him was to give Midori what Chiaki wished he had in the past. And in the future to come, when Midori gets through to Chiaki that he not only became a hero because of Chiaki, but for him—to be the hero Chiaki never had—it will be because of Meteor Impact we'll know what that truly means.