I voted for Kiyoteru.
KAITO possesses very pleasant soft voicebanks that Kiyoteru lacks, even when considering his Natural's falsetto. KAITO V1 is also very versatile, largely thanks to the flexibility of the engine itself. However, I only enjoy KAITO V3 Straight when he is tuned exceptionally well; I find his upper register to be whiny, weak, and unpleasant. On the other hand, Kiyoteru is also extremely versatile, and fills the gap in the Vocaloid voicetype lineup, it being strong tenors suitable for rock. This is important to me personally, as I am a fan of rock music and belting. If I wanted to reach for a Japanese baritone voice, I would pick VY2 or Gackpo over KAITO. Although Kiyoteru's V4 voicebanks have weak diction which make notebending more challenging, I am happier with the results I produce with them over KAITO's V3 banks. I have heard some fantastic usages of KAITO V1, but I have never used him myself. As Kiyoteru is one of my favourite Vocaloids, I chose him.