These two points are more important than university rankings for a PhD!

People often only give importance to university rankings when applying for a Ph.D.
Rankings are important, but not as important as these two factors.

1. Your supervisor. Look at,
   1. Volume and quality (citations) of his/her research.
   2. Designation and administrative roles. You'll get some advantage in funding and infrastructure if your guide is an established faculty in the college.
   3. The number of funded projects. How good is your supervisor in getting research funds?
   4. Consistency (Does he/she changes his/her main research area frequently over years?)
   5. Recent first-author publications. Is your supervisor still an active researcher? etc..
2. The overall performance of the department. Since the department is the environment where you and your supervisor thrives, it can affect your research in many ways. Look at,
   1. Recently joined faculties. For a good department, the bar for new faculties is always high.
   2. Other Ph.D. students in the department. Better peers => better suggestions, debates & brainstorming sections.
   3. Interdepartmental politics. Join Facebook groups and PM current students learn more. It's generally hard to thrive in an environment which lacks tranquility.
   4. Faculty-student ratio. This is important because in a department where this ratio is low, naturally Ph.D. students would have more teaching/evaluation work. etc..