What is the highest ranking nun called?

Answer 1

In a monastery, the Abbess is called Reverend Mother, in England, and countries that still use English social rules, then she would be referred to as Lady Abbess, and addressed as My Lady or Reverend Mother.

Answer 2

Roman Catholic AnswerThe "highest ranking nun" in a community would be called Reverend Mother Superior or Reverend Mother General. In an abbey, she would be also be referred to as Lady Abbess, or My Lady.