Superconductivity is a huge field, it involves everything from fundamental condensed matter physics, bulk applications (cables, magnets), materials research (synthesising superconducting materials etc, much of which is done by inorganic chemists), device physics etc
Most universities only have groups that are active in maybe one or two of these areas so there is no "best university" for the whole field. Also, low-Tc and high-Tc are to some extent two separate fields (the former being mostly focused on device physics).
So you have to be more specific. What are you interested in?