I was thinking that this could be Siuan. She had no need to force herself to train since she was already stronger than most of the Aes Sedai who remained at the Tower, she was too busy with politics, had access to angreal to boost her strength, and was Stilled before she was old enough that we could assume she had reached her full strength. We know much of Siuan's potential - but we never hear much about it. Her skill is said to be in managing the spy networks, gathering intelligence, and projecting her personality to get what she wants in politics.
I don't think it's Cadsuane - I think the Green Ajah would push their members to full strength moreso than other Ajahs because their members are out in the world, and the entire mission of this Ajah is to ready themselves for Tarmon Gaidon. On the battlefield, potential means less than developed strength. And Cadsuane would want maximum strength (and is old enough to have achieved it without forcing herself) - she has some emotional drive to do so from her prior experience with the old Wilder, and she would want insurance in case a male channeler or another Sister wanted to Shield her to shut her up. As a control freak, she wouldn't like that.
The most boring answer would have been Martine Jenata/Setalle Anan - she studied ter'angreal for 40 years before being burned out, so she would have held saidar for long periods of time, but she wouldn't need to exert herself to the extent that she would have in a more demanding profession. Elayne never needed to hold a large amount of saidar to study ter'angreal, so it's possible that Martine/Setalle lost her abilities before getting her full potential.
I can't imagine any Sister with any political agenda would be below her full potential. Given how important strength is to political aspiration within the Tower I can't imagine Suian not being pushed to her full strength. Not to mention her thoughts in NS about getting out from under Citalia ... motivation right there for her to develop to her peak!
Verin is also unlikely IMO as it would seem very strange for a Black Sister to have held herself back. But I agree with your rationale on her likelihood, had she not been Black I would totally think it was her.
Given the parameters we've heard, I'd look to one of the Namelle sisters who had spent a very long time outside of the Tower in retirement. It's possible one of them was actually stronger than the other and held herself back so as not to create a difference situation between them. Alternatively Samitsu or one of the other women in Cadsuane's entourage might be good candidates as they don't seem to be as politically motivated and a woman like Samitsu would rely far more on her Talent than her power level.
