I definitely agree with those saying KOKIA. I haven't heard MISIA or Superfly, but I think KOKIA's amazing. Her range is terrifying sometimes. And although it's completely unrelated, she's an amazing songwriter. It's like music just streams through her blood.
A good, underrated singer is Kanon (not Wakeshima! although I love her too). She had professional training while she lived in Western countries (and Australia) so she's really good in the technicalities. I have heard complaints about her vibrato, but I guess that has little to do with her range.
Lingering Snow~Nagori Yuki
or
Saga~This is My Road
Otherwise I think it's difficult to find really great singers in J-Pop. It's like pop everywhere, it's image that sells in the end with a bit of help from a decent voice.