Finding jobs for Ph.D graduate in mathematics is the main concern. Available options are namely in general industry and academia. I see, for instance, some Ph.D in math (algebraic geometry) working in Bank sector! This incompatibility between field of study and job might back to the rank of math department of graduated person. There maybe exists some other reasons. But I want to know about the upper bound of ranks of departments might be considered as top ranks? Additionally, which university ranking is most reliable in industry sector and in academia?
As a graduate PhD your options are staying in academia and moving to industry, as you correctly point out. If you stay in academia, be it as research staff or as postdoc, it is generally because there is a interest in your field of study. In industry however, especially with rather specialized studies, you are not hired for your research to this point. With a PhD you have proven you are capable of independently solving challenging problems. And math/physics PhDs are generally considered as being good at working with numbers and abstract concepts. This is why they end up in the banking sector or working for insurance companies. It doesn't matter if your PhD is from a high ranking university or not, once you move past a certain point of specialization there is simply no demand for the research you are working on, you are hired for the skills you developed during your PhD