Consider that you may not be able to tell whether an instructor is good or not whatever people tell you because you may have a view about your own competence. If it is PPL that you are referring to then find the most experienced person you can, that will assess you and may then even recommend an instructor for you. The best instructors may no longer teach PPL though obviously so do. Consider what kind of flying you hope to do and find someone to talk to who actually has done or is doing it. Otherwise join a club and fly with many different pilots and learn from them all. You will learn from any instructor but if your teacher can provide training in your preferred learning style you will progress quickly. Good luck!