This question is primarily used to see what you value in a manager. The best way to answer this question is to explain a little bit about the manager and list the things you respect. Also include that you learned how to become a good manager through your experience with your good manager.
This is an easy answer for me because I was truly amazed at my first manager. I made sure to explain why he was the best manager and what I learned from him. Another reason why it is important to be able to answer this question is because most great employees were taught by a great manager.