St Stephen’s Cathedral (Dom St. Stephan) in Passau is the baroque hilltop cathedral that dominates the Old Town at the confluence of the Danube, Inn, and Ilz rivers.
it has the largest baroque church interior north of the Alps, richly decorated with stucco and sculpture. designed by Italian architect Giovanni Battista Carlone, frescoes by Carpoforo Tencalla.
Here is a small watercolour detail of the rich interior
 
It also has the biggest organ!...
It is really a system of five separate organs placed around the cathedral (main organ, two gallery organs, choir organ, and a remote “Fernorgel” in the attic). All five can be played together from one central console, making it the largest Catholic church organ in the world and the largest organ in Europe, with about 17,774–17,974 pipes and 233 registers.