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.