Oliver: I don't like child voices. For one thing, I don't like the way they sound, and for another, I feel it limits what can be done with the voice, ethically speaking. And Oliver in particular is used way too much in the English fandom. A good portion of his songs would have suited another Vocaloid far better.
Avanna: Her voice is too sweet for me. I really can't stand sweet voices, for some reason. They're sort of nauseating. If she's singing really deeply, though, as in, in a male range, I can like her.
Yohioloid: He's the only post-V2 Engloid I actually like. I like how smooth he is, I like his accent, and I also like the breathy sounds he sometimes makes automatically at the ends of notes. Most people think he's muffled, and if I listen for it I can hear that, but it doesn't bother me at all and isn't something I would have noticed on my own.
Cyber Diva: I never know what to think about her. Her voice can be quite annoying sometimes, but sometimes I like it. It's really lacking in personality and character, though. She sounds very lifeless. I noticed a similar problem with Luka V4X, though Luka has it worse.
Ruby: She's okay, voice-wise. I'll admit I'm developing an aversion to her simply because of how so many are claiming she's the best English Vocaloid so far. I understand she's "higher quality", but that rarely matters to me, and in the end, I'd much rather listen to Lola. (This isn't an exaggeration. I tried having Lola sing "Hands" – an octave down, of course – and I liked it much better than Ruby's version, even without tuning.)
Dex: He sounds rather too much like Kaito English with an American accent, and I can't stand Kaito English, so... Dex isn't quite as bad as him, and does sound more natural, but he still has that sort of whiny tone. I'm also disinclined to like him because it feels like he's supplanting Big Al, which is a tragedy.
Daina: She has almost everything I dislike about Avanna, only she's not as smooth and she sounds whinier. So far I don't like her at all.
As for designs, I don't have many opinions on those. I will say that I dislike Dex and Daina's animal-ness. The designs I like the most are the futuristic, cyber-ish ones, and they're bascially the exact opposite of that. I don't like animals. I like robots.
Sorry if that was too long or vehement. I have very strong opinions...