professional education – penetration testing certifications

I have been studying penetration testing for a few months in a fairly theoretical, fragmentary and confusing way, drawing from different sources, that treat the topic from different points of view. Now I would like to start studying in a more structured way, specifically following the program of some certification. If it were possible, it would be very useful to me having a list of the best known certifications, from the simplest to the most difficult, to set up a course of study. Could you also tell me, from your experience, which are the best sources (books, tutorial, online courses, etc ..) from which to study for each certification?