Well, obviously I can't start my coment without saying: CUUUUUUUUUUUUUUUUUL!!! SHE IS SO UNDERRATED THAT EVEN YOU GUYS FORGOT HER, THAT'S SO UNFAIR!
Kkkkkkkkkk just kidding, but I really think CUL is underrated. I also agree that Will sees way more love than the blonde one (He and Kyo are personaly my favorites and I love them), so I will also mention him as an underrated vocaloid VB.
I totally agree with all Ceviloids and Synthloids are very underrated (Kevin and Rose, my beloveds! T3T) so I won't mention them besides these two.
Tbh, for me the most underrated voicebanks are: Yumemi Nemu, Macne Nana, Rana, Unity, Arsloid, Mew, Miki, Aoki Lapis and Merli. I'm not a huge fan of the argument "Urr durr, this voicebank is hard to work with, urr durr!" because expecting all voicebanks to be the same in usage is kinda... not cool and doesn't make sense at all XD But even thinking that way I unfortunately have to agree that not all "underrated" voicebanks has quality enough to be considered an "underrated" vb (I know, I'm a hypocrite XD).
So, by using the logic of "high quality vbs that aren't used as much as they deserves" as I way to measure who is underrated and who is just not a good vb to work with, I have to agree that just those I mentioned are really underrated gems. Specially Unity-chan that looks so high quality that she seems to be just ready to be mixed, while other voicebanks you need to do some vocal proccessing to make them sound cool.
But if I didn't go by this logic I would say anyone except for Miku, Luka, Kagamine Twins, Kaito and Gumi deserves way more love. And if it was just by "how much fans exists in their fandom" I totally agree that the real underrated gems are Chika, Mew, Arsloid, Unity-chan and Merli (But specially Chika and Mew). Bro, no one talks about them ever. I'm so happy I found CatYss channel that uses Chika a lot, hope I see a Mew channel too! *3