My two guesses are Moiraine and Verin. Perhaps Moiraine didn't reach her full potential prior to her encounter with the Snakes and Foxes. Now, she will never reach her full potential because she has been lowered.
I have a suspicion about Verin, but it is little more than suspicion. Perhaps something with her experimenting with the Ring Terangreal. She indicated using it left her with a headache and unable to channel for some time.
THoughts?
Moiraine - I doubt it's her. Her mission seems to have required her to be more aggressive in the accumulation of power than other Sisters. She explicitly says she has gotten more dangerous the next time she sees the party, and I doubt it's just because she learned balefire. It was probably because she realized she needed to be as powerful as possible to maximize her use of that and other weaves to protect Rand, and I think she would be more likely to channel enough to reach her full power ahead of Siuan.
I wondered about Verin, as well - but she's another character whose personality, and loner tendencies, do point to her maxing out her strength. But going the other way, the best way to stay out of being ordered to do too much B.A. business is to keep her profile and level of strength low.
