yes maybe keiko is the most flawless.
however, value of a singer is not only accuracy, you know. everyone has their own appeal.
keiko's singing is full of elegantly-controlled sentiment. her voice makes me excited and thoughtful at the same time.
wakana's singing is dreamy, melancholy, nostalgic and clear. she's my favorite.
in my opinion, hikaru is like a shaman - when being 'possessed', her performance tells something holy. I've been moved by it many times.