Favorite Cathedral in Europe
The Koelner Dom is hands down my favorite cathedral in Europe. You really cannot believe your eyes when you see this structure, both the size of it, but at the same time the detail of the stonework as you walk around, for example the statue carvings over the doors, and generally all of the age, history and Gothic architecture of the Dom. It's really great. At night it is lit up, and just dominates the Cologne skyline, with particularly scenic views from the other side of the Rhine. We toured inside, and climbed the stairs to the tower, and these were cool, but to be honest probably the bigger point for me were the exterior views which are amazing. Really great, highly recommend this if you are making a visit to Germany.
One other note, if you are in this region, if you travel 90 minutes or so south of Cologne to the Rhine valley, that is really picturesque also, including a number of castles and vineyards as you go through the valley (you can go by train, boat or drive). That's maybe another quick recommendation.