If we're not being at all restrictive about it, I would have to say Hisako Kanemoto. She is the voice actress for Ika Musume, and has done roughly 12 image singles for her character. My favorite song from her would be Shinryaku no Wa wo Megurishi Unmei no Shito...ja naika
. Squidy puns are life~
If we're going for more mainstream, I might say Minami Takayama (the vocalist/composer for Two-Mix). My favorite song by them would probably be Last Impression
. Nami Tamaki and her song Realize
is also fantastic.