What is the measure of a successful college professor? Do they provide the 'easy' A's or challenge the students despite grades?
Successful is subjective, but let's look at who would be evaluating the metrics for success. The faculty/administration of the University would deem a faculty member successful by meeting or exceeding the requirements for tenure: instruction, scholarly productions, and service to the department, School/College and University. The faculty member maintained a professional reputation, timely with submissions and prepared their classes for the next course in the major department with the desired baseline of information and foundation to navigate the next class. Took on the responsibility of advising students at all levels, while conducting research and being awarded research grants and/or awards.
The students would want a faculty member who provided a clear outline of the course, grading, and communicated effectively the desired learning concepts for students to be successful. Most students want a good grade in the course, but don't want to be in trouble for the next course. The better prepared students want their efforts rewarded with higher grades, while the slackers want to be able to miss lectures and still master the material. They want a mentor who is patient and available during office hours, and spends more time going over exam material before the test, instead of after it.
The hardest evaluator would probably be the Professor. Many a doctoral student had visions of how their career and life would play out. Few are "surprised " that even more compromises are made after receiving the PhD than before it. How hard to clean up and publish their dissertation work, to find a position or do a post-doctoral research based on the field. Wonder why they have to struggle to find a tenure track position, or that it comes after working as an Adjunct or Lecturer for a number of years. That your employment becomes a variable in any relationship. That your demands for professional achievement battle with family and personal goals like everyone else. That you may never have the financial success of some of your friends, colleagues or students; and that is fine. I hope this helps. Good luck!