Hmm... Hmm...
I'd have to say Dawn from Pokemon. Period. Voice-wise, appearances, etc. I just can't stand her D=<
Oh, and when I liked it, younger Mitsuki from Fullmoon wo Sagashite. She was such a whiner, and HAD to get things her own way. She might've supposedly had a year left to live, but still... I just couldn't stand the sight of her >.> The older one was fine, but that's just 'cause Changin' My Life's a cool J-Pop group, and it is myco (the lead singer of CML) that voices Mitsuki in both forms XD