Really it's tough to go wrong with Christmas Markets in Germany. They're all great. My top recommendation though if you only go to one would be the Stuttgart Christmas Market (
here's the english version of the page on the Stuttgart tourism site). They even have a 360 panoramic to check out (though I prefer the Destination360
Kris Kringle Mart pano which was taken at night). Most of the market stuff opens towards the end of this month (November) and runs til a couple days before Christmas.
But really, most likely you'll be happy and impressed with the Christmas Markets in any of Germany's bigger cities. Good luck and have fun!