A good teacher always hears whatever he is telling in front of the students. So he is teaching himself.
Students might be hearing 50-70 % of whatever the teacher told. But the teacher would be hearing 100%, assuming himself to be dedicated upon the subject.
He becomes the best teacher if he could motivate his students to generate curiosity and desire to learn on their own.
He may not be able to answer the entire questions asked by students on the same moment. (He would be answering after referencing if couldn’t do on the spot). But even then, I will rate him as the best teacher as he could make the students to think and raise questions from their curiosity.