Whenever you’re crossing the Dom square, you can watch people trying to get a good picture of the huge church with everything in the frame.
Spoiler alert: in most cases they don’t manage to do it…
The best thing for a clear view, and not only of the Dom, is to go up a tall building.
But there aren’t so many of them in Cologne. The television tower is closed due to a state of disrepair and the tower of the Dom is quite hard to climb : 533 steps in a very small staircase and in summertime very overcrowded and with muggy air…
So where can you go? Again, to the other side of the river Rhine! From there you have an excellent view over the old city and the Dom. For example, from the Rheinboulevard.
But you can go higher: on top of the Köln Triangle!
So just take a lovely walk over the railway bridge (Hohenzollernbrücke) to the other side of the river to the Köln Triangle (also reachable by public transport/car/bike from the other side). Find the entrance to the observation deck (Aussichtsplattform) on the right hand side (coming from the bridge).
Having reached the top, you will have a fantastic 360° view of the Dom, the old city and the other areas of Cologne and the scenery around.
The platform is fully secured with big windows on which there is information about what you can see.
The best view of Cologne in my opinion!