BEYOND by niki ft Sekka Yufu would be a good song for a soft or whispery voicebank. If you use the soft VB with a more powerful one then Kuroneko by Akagami and Raptor would be a good song, especially if you want to show a power comparison with the soft VB.
UtaForum.net
CrimsonNeko
Tomato Hentai
End of Rain. I would tell you more but I can't think of any others off the top of my head.