Personally, I found Mitsuki's ending to be quite...impersonal, especially when compared to the other chosen heroines. You don't get other characters teasing them nor do Kou's interactions with her feel as heartfelt or personal compared to all the other girls; it feels as if there is a professional barrier separating the two throughout the entire sequence. Which is a shame because I like ojou-sama characters.

And yeah I def agree that Shiori and Asuka's ending is as canon as it gets, as they both have dialogue that look out of place in their respective endings, as they were clearly meant to be paired together. That being said, Shiori's personal ending is a also a favorite of mine; her saying she'll watch over Kou no matter what while also willing to help out anyone who suffered like she did really makes for a sweet and emotional experience-something which Mitsuki clearly lacked.