This is a good although old list conducted among Japanese music critics. Some of the newer artists should move ahead in the list:
http://www.hmv.co.jp/news/article/311040091/
This sounds about right. The most iconic female artists are: Yumi Matsutoya, Hibari Misora, Momoe Yamaguchi, and Miyuki Nakajima. Right now, I would put Dreams Come True above Utada, and Namie much closer.
Let's be honest. Artists such as Nana Kishino have strong fan bases, but to be iconic you have to appeal beyond a fan base. Be part of the overall musical conversation across generations and genres and not be limited to foreign fans, otaku fans, young fans, etc. Some people are too young to have historical perspective, or too ingrained in otaku subculture to be objective. That Music Station list is skewed toward recent releases, so I doubt it's reliable.
Another list: Rolling Stone Japan's Greatest Rock Albums:
http://exclaim.ca/music/article/fina...ms_of_all_time
Some pop albums in there. Very good list, although it might appeal more to rock fans than pop fans. Many iconic albums, but the list includes brilliant albums that didn't achieve iconic status (Supercar, for instance).
I'm trying to find another list of most important Japanese vocalists.