Ms. Keller had two key incidents that led her out of her “prison”. She had been very frustrated, because she couldn’t communicate, and it had led her to naughty behavior.
First incident was the one with water. She hadn’t been able to understand what it really was, until Ms. Sullivan poured water on her hands. Since then, she knew what it was, and it opened her mind. She got happy and freed.
Finally, the other incident was when she climbed the tree. Nature warned her of its threat. She became frightened about that incident, but later climbed a tree again and it freed her.
In conclusion, I believe that the most important incident was the one that had to do with water, because that for sure freed her.