I used to have a bit of spite for Kagamine Len, Fukase and Utatane Piko, because of their unrealistic portrayal of higher tone male voices, Len being the worst because of him not actually being voiced by a male.
I think these "shota" voicebanks were damaging to the perception of a male vocal range.
But if there's one vocal I actually don't like listening to, it's Arsloid, because of his execution of "extra" vocals feeling like a cop out, his incredibly nasal, buzzy tone whenever a note is just a little out of talking range, his quiet lower range that sounds muffled, and his complete inability to make pitchbends sound expressive.
Who were they trying to appeal to? Arsloid is a master of nothing.
Kiyoteru as a V2 can surpass Arsloid in every way.
As for not the "least," I do have some criticisms regarding other vocaloids.
Oliver creeps me out as a concept (fittingly, considering what most of his songs are about.) and sounds so unintelligibly mumbly he might as well just be a sine wave that occasionally clicks for consonants, although that's a common problem with most English voices of the era (looking at you Cryptonloid English banks...)
Shiki Rowen's demo gave off a horrible first impression (besides the whole RVC thing that happened with it...) because of him being forced into a female voice range that he clearly cannot maintain, making him sound strained, V6's pitch model did not do him any favors, he cannot hold a single note in that entire song. Since nobody is willing to use Shiki Rowen (understandably so,) I cannot find any decent uses of his V6 bank.
I cannot hate Gachapoid simply because no one uses him for his intended niche, he is instead memed to death with songs clearly not designed for a cartoon voice.
I guess I should end with at least one canonically female character, although this one is kind of super-specific.
It's not that I dislike Hatsune Miku, it's just that whenever people do the "yowane haku" style voice using a XSY of Dark and Solid and the GEN parameter set to a male value, it sounds overly processed and unnatural, which is a shame because these banks sound great by default, without eachother.
Lily or can do this "emo" role better, so IMO it's not worth it to force Miku into a deep voice that she isn't.