Of those you listed, I'd vote Budapest as #1, Munich as #2, Berlin as distinctly #3, and Salzburg as a very very distant #4. I can't speak to the others on your list that you hadn't already nixed. The only other Gerrman or eastern European cities I'm familiar with during wintertime would rank below Berlin, although I'd imagine there are several smaller cities that would be amazing.
Budapest:
I spent almost a month in Budapest in late December to mid January. I love this city and surrounding communities. Best history museums you'll find in eastern Europe, and it's absolutely beautiful. It has the architectural beauty of Prague but adds the georgraphical beauty of Buda. I might also describe it as a combo of Prague & Vienna, but better than both.
I think Budapest does a great job of maximizing the winter beauty of the Danube a litte more than most cities do with their rivers. Christmas Markets are some of the best in Europe.
Budapest also has thermal baths that are hot all year long. Thermal baths when it's 15 degrees outside is an awesome experience that you won't find elsewhere in Europe like you do in Budapest.
And the bars, restaurants, & nightlife are excellent & diverse. Something there for everyone regardless of what suits you.
Munich:
Bavaria is my favorite part of Europe. I've never been there in December, but I love German Christmas Markets and eastern European Christmas markets, and I've heard the Bavarian Christmas Markets of Munich are excellent. And the beer & beer halls are excellent any time of year.
As great as Munich is, the surrounding areas of Bavaria are even better. If you enjoy skiing & have the time, you could head south to Garmisch-Partenkirchen.
A snow-covered Neuschwanstein Castle would likely be one of the better photos you might ever take, and Linderhoff Palace would have to be beautiful this time of year.
I bet Partnach Gorge would be fascinating when it's below freezing, assuming it would be open.
If you have even more time & the weather permits (although it often doesn't during the winter), you should go to the Eagle's Nest. The Eagle's Nest covered in snow & ice is arguably the most beautiful thing I've ever seen.
Regardless of where you go, make sure they have great Christmas markets, parks, and of course the river...especially for a winter visit. Also, try to find a city with an outdoor hockey team. Even if you hate hockey (as I do), watching a minor league outdoor hockey league game is definitely a unique experience & something you'll remember. I got lucky enough to stumble upon one with an hour or two to kill, and I'm glad I did. It was weird, rambuctious, and awesome.
If you're planning to be there during Christmas, keep in mind that nearly everything in every city across eastern Europe shuts down early afternoon of Christmas Eve and doesn't reopen until the 27th. And I mean everything. No food, no stores of any kind, no transportation (maybe a limited amt on Boxing Day...but mostly none). Budapest even shuts down 14 of their 17 hospitals & clinics during those 3 days.
