European Court of Justice
The European Court of Justice - ECJ (very technically part of the "Court of Justice of the European Union") is the supreme court of the European Union, whose task is to interpret EU law and ensure its uniform application across the Union and its member states.
This includes data protection law as the GDPR, an EU regulation, is directly applicable in all member states without any need of a transformative national act.
The European Court of Justice is known to interpret the GDPR in a way that upholds the rights of data subjects and protects personal data from misuse. This led to a series of spectacular verdicts invalidating the Safe Harbor and the Privacy Shield transfer mechanism in the Schrems Rulings Assessment.