Angel really inspried me to do this so I guess I'll list who I like, who I'm mixed and who I dislike
YANHE: Shocker. My favourite since her release like two years ago. Though her pronunciation isn't the clearest, I find it much clearer than Tianyi's, I find her voice to be very unique and versatile in my opinion. Also, I'm a sucker for short haired girls.
Kiyoteru: I like his voice. I find it to be very clear (at least to my limited one-semesters worth of Japanese knowledge), especially in his V4. A lot of people say he sounds like KAITO but I don't really see it.
AVANNA: Holy mother of ZeroG, her voice is heavenly. Normally, I wouldn't like her kind of voice type (soft, "folk" so to say) but I suprisingly like her a lot. Not to mention she's pretty clear when tuned correctly.
Dex: Clear. I don't really have much to say other than that. I can't really describe why I like his voice, I just do.
VY1v4: I like them, shocker.
Mew: I also like her, woah.
Bruno and Clara: Pretty understandable for the first of their language. Lots of potential.
Iroha: Her v2 design is... Odd... But her voice is perfection. Very clear (at least in my opinion) and her v4 made it better.
Kokone: I never really liked her, but I never disliked her either. I mean, her voice was clear, and I liked one song from her, but her voice was boring, soft. And her design felt incredibly forced.
Chika: I like her voice, but it sounds so much like other vocals I can't help but put her in the "meh" section. Her design is pretty cute though
Gumi: With like, so many vocals from Internet Co. that sound like her, I'm tired of her. But her English is decent compared to some others. ahemmikuahem
Luka: shocker. Let's be honest, Crypton really screwed her up.
Cyber Diva: Lots of clarity, not enough originality
Luo Tianyi: She is my baby because she is the first Chinese vocal, but unfortunately she is complete hopeless to try and understand without lyrics. It sounds weird, like she's trying to talk through a wall and has a lot of mispronunciations. And her voice is a little too high for my tastes.
CUL: I like her, but she's nasally. And I haven't heard any songs from her in a while
Ruby: I understand Cyva is also nasal, but at least I can understand Cyva most of the time. Not to mention with all that drama, I fell out of supporting Ruby. Also, as unfair this is to Ruby, I dislike Misha. But that's a story I won't go into.
Daina: Clear, but nasally.
Miku: Ugh. Where do I even begin? Her original v2 was pretty bad, but I like her dark append, especially in slower songs. But Crypton completely ruined her English. Like YANHE or Tianyi, almost impossible to understand without lyrics. Only, I feel that YANHE and Tianyi's company at least tried. Maybe that's because they didn't know if they were going to sell or not.
Rin/Len: They both have the same issues, so I'll put them together. I feel that, like with Miku English, Crypton didn't really try with them knowing they were going to sell. Their v2, even with act 2, was kind of low quality... I once had a girl argue with me that it was the "technology of the time" was the reason why they were for LQ. I had then pointed out that Sweet Ann, who was released as the first v2 and before the Kagamine's was quite understandable, and high quality.
All in All
I guess you can see I'm a sucker for clarity. (ironically though, YANHE my most favourite vocaloid of all time isn't very clear and had her background issues) And usually I think it's the effort a company puts into a VOCALOID that explains the quality (with the exception of the V1s, because the VOCALOID technology was still early), but meh that's just me.