Mostly based on chara designs since I don't really listen to Vocaloid music that much.
S: Rin, Len, Hime
A: Mikoto, AVANNA, VY1, IA, Yukari
B: Miku, Luka, GUMI, KAITO, MEIKO, Fukase, VY2, Flower
C: Oliver, Una
D: Gakupo, every Macne
F: Gachapoid, Kaai Yuki
Would get a C if I judged only on...