Honestly you can't go wrong with any Christmas market- they all have great food and plenty of Glühwein!
As far as the best one are your area, I'll join some others and mention the Stuttgarter Weihnachtsmarkt as my favorite. In many of the big markets (Dortmund, Freiburg, etc etc) the market is scattered throughout the city, but in Stuttgart it's all concentrated in one area right under the Rathaus...it's great.
From FRA you can get there in an hour and fifteen minutes with an ICE train from DB. If you buy in advance (especially on
this site) you can make the journey for less than 20 euros.
Enjoy!