I don't think that any of the super-strong women are obscure, so I don't think a woman like Norla or Sera are possibilities. If RJ did make them so strong he would have inserted something about their power into the story, much like he did with Someryn.

Someryn has two quotes that suggest that she may be this woman. She has never met a woman who is remotely close to her in strength and is very irritated that Graendal is stronger, and Graendal herself considers her to be 'not negligible', which is a high compliment indeed from a Forsaken if you see how Moghedien appraised Nynaeve's ability.

Anyway, I'm not going to defend the point vigorously. The other five women have strengths that are all but confirmed to be greater than Nynaeve's, and Someryn can be added as the sixth with two quotes that make her a forerunner for this position long before any other channeler can be placed there.