You can find a survey among all AKB members regarding this on AKB fashion Book 2009 or something: Masuda get most votes which count around 50%, takamina seconded to yuka while nontty (!) taking the third place
I'm sorry for takamina and her supporters but I DO feel her vocal ability is overrated for some reason
My ranking at this moment, seems about:
Masuda
Iwasa
Katayama
Takahashi
Akimoto
Sato Natsuki
For Warotta it's a bit difficult as barrierV explained, also their voices are too much "decorated" lol. If I have to take a guess, I would say Oota>>>Kikuchi>Watanabe>Nakagawa>>>>Hirajima