The question in the title is immensely subjective and broad, and so I would like to narrow it to an answerable question:
What measures are in common use by administrators and researchers in determining the quality of an instructor's teaching? What, if any, is the correlation between these measures?
I ask because I've had difficulty answering this question for myself. I originally thought that test scores would be the best answer, but I have seen the same teacher have the calculus section with lowest scores in a department and the calculus section with the highest scores in the department in the same semester.
I have also seen teachers that I would identify as good teachers have bad test scores, and vice-versa.
I've had theories as to why test scores may not be correlated to good teaching; for instance, poor students may prefer a good professor, and may be interested in joining such a professors class (this effect could be determined by comparing pretests and posttests, but I have not carried out this analysis).
This has led me to wonder if "good teachers" have any positive effect on their students at all, compared to "average teachers". That's why I have asked the highlighted question above; I would like to find out what others have looked at when evaluating teaching, to help me understand what a "good teacher" is.