When conisdering to do a PhD what is more important-a supervisor who seems very keen about the subject or the university ranking? Is it really necessary to see more than one supervisor or a univeristy while applying if you already found a supervisor with a shared view into research?

This is a commonly discussed question, you might want to search the archives. To summarise, the reputation of the supervisory team is far more important than the overall reputation of the Uni. However, the most important criteria are; do you like the project, does it inspire you? Do you like the supervisor(s) and team? Can you see yourself (happily) living and working in the place for at least three years? Good luck!