No doubt for PhD - University ranking and for Postdoc - research group
All the best
In Researchersjob you can ask and answer questions and share your experience with others!
For applying PhD, according to priorities you should choose
First - Reputation/Ranking of University
Second - Doctoral Advisor/Research Group
For getting PhD degree, considering university ranking is very important because after PhD also you need related jobs or postdoc than ranking will help you.