Helen Keller is the miracle because she finally becomes able to transcend the dark world in which she has been trapped. Annie is the miracle worker because she is the only one who has been able to reach Helen and transport her out of her dark desperation in which she cannot understand many things.
At the water pump, Helen miraculously recalls her babyish attempt to say "water" and is able to connect the object with the letters that Annie spells into her hand. This connection is not only a miracle for herself, but she provides her teacher the miracle of being able to love someone again without fear. In the final scene, after Helen has discovered the connection between letters and objects, she goes to her mother and asks her the word for "Annie" and then strikes Kate's pocket for the key. Afterwards, she carries the key to Annie; Annie spells "I love Helen." Helen has helped Annie to heal the wound in her heart left by the death of her brother, a loss about which she has always felt guilty.
Later in her life, Helen Keller recorded these experiences of her awakening into knowledge and Annie's being able to love again:
When we do the best we can, we never know what miracle is wrought in our life, or in the life of another.
These words are the inspiration for the title of William Gibson's play The Miracle Worker, in which a nearly blind Annie Sullivan finds her purpose in life, and Helen Keller forms a lasting friendship with the woman who has opened the world to her.