Cologne might be an idea, but do not expect a big firework. It is the same as in Berlin: Everyone will do their own fireworks.
A nice view of the city will be from the Hyatt in Deutz, from there you will see the "Dom" (cathedral) and the "Altstadt" (old town) across the river Rhine with all the fireworks. And Deutz has some nice restaurants and public transport options.

But do not rule out Berlin as there are some nice areas there (Prenzlauer Berg, etc.), that are not as crowded and rowdy as the area around the Brandenburger Tor on NYE.
Another option in Germany might be Hamburg or Dresden. And, if you want it less crowded, maybe a smaller, picturesque towns with good connections to an airport like Heidelberg, Mainz, Bremen, ... it really comes to what you want.

My favourite place to be (apart from Cologne, but I am biased about that) is Salzburg - take a nice hotel with a view of the city and enjoy the scenery!

If you have any question about Cologne, please send me a pn.