I would have said Houseki, but I litterally just watched this live performance of Mizu no Akashi, and it seemed like Wakana was going to cry, which brought me near tears as well. So I'll go with Mizu no Akashi.
keiko. and clara kennedy. she could sing Kajiura's English songs like sweet song, maybe tomorrow, a duet with Keiko in I swear (dayum), fake wings, and so on.
Kajiura song that you can't stand? or the song that you always skip whenever your music player plays it.