University ranking is not as important as supervisor quality. hD students normally follow one or two professors to do high quality research. University ranking does not help PhD graduates much. If you can follow Big Cow to do high quality research in a low rank university, your future will also be brilliant. Following good and active professors can lead your work to be published. The quality of published papers is the most important factor for the job hunting success. Choosing universities should also be based on your research interests. Does any professor do what you are interested? If no, mismatch happens. Even though you study in famous universities, you would be unhappy.