I’ll start!

Vocaloid: Flower v4. I love her growls

SynthV: Kasane Teto. I may be very biased here, but I really like her original songs.

CeVIO: RIME. Her breathy voice is cool, and is used especially well in Utsu-P’s collage.

Utau: Anna Nyui. A lot of options, and a pretty versatile voicebank I love to use.