[ayumisrael]

From Japanese perspective, Andrenekoi is right, from foreign jpop listeners precspective, they choose to put namie over ayu because of her 2nd peak and that she is the most popular solo female now still even though she slowly declines as well, but I think that both sides forget that maybe ayu's peak around the A BEST era was something that even namie never got to, it was like ayu and hikki were the legend-est solo females ever and no other solo female reached that level other then... ayu and hikki. Also ayu declined really slowly, even though her last 3 years were a bit of a commercial mess, the woman sold over a million per original/compilation album from 1999 to 2005 and then over 500,000 to 2008, 9 years of being on top/remain close to it being active on successful shows and great sales all of that time and even on 2010 ayu was one spot after namie as the best selling artist of that year in overall sales. She declined REALLY slow and kept selling high, unlike all other solo females, that's why I wouldn't rush to put namie over ayu.

Even though it's hard to compare ayu, hikki and namie, there are the older solo females like seiko, yuming, akina, momoe, misora and probably a few more. But I'm not sure if they reaced the level of the other three.

What is sure, people shouldn't throw ayu under namie that fast, because second peak, more relatable by the public, and more popular than all other solo females, I still don't think namie reached the ayu A BEST-ayu vs. hikki level in both her peaks. So, each of the three has their share of legendary stuff.
hikki main point is having First Love and Distance, mainly so I don't think ayu and namie both should be under her as well.
Since hikki is not selling anything these years (not even all that much from her First Love 15th anniversary edition), I would say that even ayu and kuu are more successful than her now, because at least they sell a couple of ten thousands, and make loads of shows.