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!
For context, I'm not someone who makes music or uses voicebanks. I just listen to a lot of music (both synthesized and not).
I'm a bit typical I guess, because my all time favourite would be Hatsune Miku. She may not be the best from a technical perspective, but she has a charm to her voice and she has a lot of good songs (which may give me some bias). And the append design is great, I love that type of cyber look.
Otherwise, I also quite like VY1, Eleanor Forte (SynthV), Ruby, Lily, Cyber Diva/Songman, Mo Qingxian, SeeU, and Megurine Luka. I like Gumi's voice but somehow just can't stand her design. I remember I was quite fond of Ring Suzune's first demos, but they completely changed her sound before release and I kinda forgot about her after that. I also liked Sachiko, especially when used classically like in the demo. Tonio was also quite interesting (altrough I only remember the original version).
Sinsy is probably the most uncommon one to hear nowadays that's on my list, but the voice is just damn impressive. Even now some of those old songs sound more realistic than those from newer engines. Just listen to Magnolia for example.
Utau I never liked to be honest. The output quality is just too low for me, even on well made voicebanks. I rather go all-in bad like using Microsoft Sam, or have more fluency. Utau is too much in between for me. But then, that's just personal taste really.
But to be honest, within certain limits production matters more to me than voicebanks itself. I rather have a meh voice in a well produced song that's my taste than my favourite voicebank in a meh produced song. Perhaps that's also why I can't pin down to 1 voicebank that easily.
I preffer proffesional productions, and am not very fond of things that sound too hobby-istic. In terms of songs I tend to like the classics (like Miku Miku Ni Shite Ageru), or very electronic upbeat songs (with that I mean songs like Hibikase, 39 Music!, Sweet Devil, Party Junky, and Cosmonauts).
It also doesn't help that I stopped following releases somewhere during Vocaloid4 (maybe early Vocaloid5, it's been a while) and the very beginning of SynthV.
But to be honest, within certain limits production matters more to me than voicebanks itself. I rather have a meh voice in a well produced song that’s my taste than my favourite voicebank in a meh produced song.
I totally get it! One of the songs I really like was made with a voicebank that is similar to Defoko's (Adachi Rei) and although I don't usually like voicebanks like that, the production really made me appreciate it more: https://youtu.be/1G4bYPl09Zo?si=uPzd4yormh2NjU01
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