If you do happen to find yourself in Munich in December, these are my tips for which markets to visit. I'm sure there are tons of others that I have yet to discover, but here's my current shortlist of go-to spots:
Best Overall: Middle Ages Market
Wittelsbacher Platz (near Odeonsplatz)
This market has a great old-time flare, including performances by jugglers and dancers, and music dating from the middle ages. It also has a Feuerzangenbowle (Glühwein on fire - not to be found at all markets). The stalls sell a lot of handmade goods, and food is also appropriate to the time period, e.g. you can walk around the market while gnawing on a turkey leg.
Best Late Night: Tollwood
on the Theresienwiese
This is like a combination of Christmas market, hippy market, music venue, and just a full-on winter festival. There's a big tents with art exhibitions, an international food hall, and a big bazaar. It's probably got the best shopping of any of the markets, though not always the most traditional offerings. And you're bound to find a lot of hippy imported goods, like statues of the Buddha and incense. One of the great things about this market is that if it is cold or raining/snowing, you can go into the tents and escape the bad weather. Like the Middle Ages Market, there's also a Feuerzangenbowle.
Most Picturesque: Munich Christkindlmarkt
This is the official Munich Christmas Market that is located on the main square at the Rathaus, and flows down the shopping street in the direction of the ice skating rink on Karlsplatz. Photos of this market are gorgeous, with the city all lit up with twinkly lights, and the Rathaus and its famous Glockenspiel hovering over the market in the background.
Best Neighborhood Picks:
Schwabing, Münchner Freiheit
All of these are great markets that have a real neighborhood feel to them. People meet up here with their neighbors and friends for cup of Glühwein after work, or on a weekend with the whole family. You'll see lots of kids running around and you're more than likely to run into someone you know.
Most Colorful: The Pink Market
It's a tiny little market put on by the gay and lesbian community in Munich, and it's open later than many of the other markets (past 8PM! Woohoo!). There are also performances every evening at 19:00.
Most Romantic: Rothenburg ob der Tauber or Dinkelsbühl
To really get the authentic Christmas Market experience, you should leave the city and head for a smaller town. My recommendation is to check out a couple of markets on Germany's romantic road, in particular the markets in the medieval towns of Rothenburg ob der Tauber and Dinkelsbühl.
Best Day Trip: Chiemsee Fraueninsel
This is an island Christmas Market in the middle of the Chiemsee. This small island is the home of a cloisters and is beautiful all year round, but in the winter it is magical as it is lit up with white lights and the whole island is converted over into a Christmas Market. You have to take a boat a boat from Prien am Chiemsee to get there.