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 like the robotic sounding ones more than the realistic ones. It very much depends on the tuning but V2 Miku/Luka/Gumi are alsmost always great