Well we have Lanfear, obviously. I always assumed Graendal and Semirhage were stronger than Nynaeve, and Moghedian is equal to her. Not sure about Mesaana. There's Sharina and Alivia. That leaves us with one space.

It seems most likely it's Talaan - she's young, and though she's about equal to Nynaeve at the moment, she's surely got more strength to gain.

If Talaan isn't one of the six, or I've overestimated the strength of one of the forsaken, it's possible you're right. I wish RJ had dropped us a few more hints about Norla. Cadsuane spoke of 'earning' her ornaments, which doesn't sound like it was something that happened overnight. Did Norla imprison her, somehow? Set her tasks? Why? She wouldn't necessarily have had to have been stronger than Cadsuane to do it - just look at that kinswoman who can weave the unbreakable shields.

My own little pet theory, which I admit is extremely unlikely, is that the sixth super-strong female channeler is Sera, one of Tuon's damane. She's mentioned as being 'the strongest', nothing else, and Tuon did have a whole continent of channelers to choose from. But there doesn't seem to be much point to having such a strong channeler and giving her no role.