Charlotte from IS <Infinite Stratos> is unbelievably cute. And being voiced by Kana Hanazawa doesn't hurt, either.
Elsie and Kanon from The World God Only Knows are both really cute characters. They also happen to be voiced by Kanae Itou and Nao Toyama, respectively; one being a favourite established voice actor of mine and the other my favourite new voice actor.
Winry from FMA. Not much to say about this one, but she speaks for herself.
I have many more, like Hinagiku from Hayate no Gotoku and Mugi from K-ON!, but that should do for now. Definitely Charlotte at the top right now, though.