Basically any sisterly scene between Luca and Cloche.
As well as any scene between Cocona and Croix.
In Jacqli's ending, Croix confesses his feelings with song, which later turns into a duet with Jacqli. Note that this is the only song (in the games themselves) with a male singer which makes this event even more significant.
Reisha's lullaby and what it stands for.
The lyrics themselves describe the bond between the Maiden of Homura/Fuero and the Maiden of Mio/Aqua.
Luca's late birth-mother Arshe taught the song to Reisha, her close attendant. It ends up passed on to both of the Maidens from both of their mothers.
It calls back to the happier times of the Trulywaath family.Kinda makes you turn on the waterways when you see their early childhood memories in the final level of the Infelsphere...
Combined with Tear Jerker, Reisha's second and final song Eternally Bonded. It has no effects (Reisha is human, thus it's not true Hymmnos), but like the above example with Croix, it's sung overflowing with feelings of love, sadness, and happiness, in hopes that that her daughters would understand it, and in Luca's case, forgive her for her neglect.
Try to listen to the song's two final lines and read its translation without crying... Oh, life that was given birth in a shining day, your gentle smile will never disappear, and it will bloom eternally...
With the mechanics of Hymmnos in play, the fact that two humans, Reisha and Croix, use Hymmnos to convey their feelings the way Reyvateils do. In a technical sense it shouldn't be effective, but the effort they put into learning the language and utilizing it for others is what makes it powerful and touching.