Hisame remembering the moment she decided to do her best for Sami.
Hanako's flashback monologue of her relationship with Tarou.
The last page of Chapter 62, when Samidare admits that she loves Yuuhi more than she does the Earth (this makes a lot of sense in context). If this doesn't bring a tear to your eye, then you have no soul...
Chapter 63. All of it.
The ending of the series. Easily one of the best endings to a story such as this one.
Adding to the above, the moment when Samidare gave them all hand-made plush dolls resembling their animal companions.
The three most romantic moments in the series are what amount to two promises of mutual suicide, and a promise from one to commit mass murder on the other's behalf. Each one manages to be somehow heartwarming, chilling, and sweet all at the same time.