In what act is the best character expression descibed?
I would say it is in Act II that we discover the tenacity of both Helen and Annie Sullivan. Their wills clash violently; Helen is used to getting her way, and Annie is determined to teach Helen to communicate more effectively. Until she understands words equate with objects, she argues, Helen will never be more than a needy animal.
In this Act, we also learn much more about Annie herself, through her memories of her own horrors of the orphange where she spent her formative years. The traumas she endured help Annie empathize with Helen. I think her singing of the lullaby to the unhearing Helen at the conclusion of this act is almost as powerful an expression of character as is Helen's first communicated word, "wah-wah" (water, Act I).