This is an important question and I believe it changes with time. As a teacher educator for over 30 years I have been witness to many different kinds of 'effective teachers.' I suppose if I could extract one quality common to all it would be their eagerness to learn; to wonder alongside their students.
Agreed with other scholars' comments on the qualities of a good teacher. For me also want to add "passion" - because I encountered some teachers who were passionate to teach / share their knowledge / experience unreservedly which is contagious & has lasting effect on their students' learning & doing.
I think the foremost quality of a teacher is their command on the subject.They support their students to learn something new every time. Moreover with whom the student can share their problems as they are the source of inspiration for their students.