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.
I’ve only played with the first sets of Vocaloids at the time, and Kaito was my favourite. I like how he gained popularity. I have a cute Nendoroid of him as well!