Christmas Markets in Germany - Suggestions

...perhaps stay in Munich for a few days as a base for seeing the markets
accessible by train.

FRA will be your in-and-out airport, right? Unless you have specific sightseeing in mind BEYOND the Christmas markets, I see a trip to Munich as time-eating ground travel for no real purpose. Christmas markets are abundant within a 1-2 hour radius of Frankfurt... Cologne, Koblenz, Mainz, Wiesbaden, Rüdesheim, Marburg, Seligenstadt, Heidelberg... It's not as though Munich has a corner on railways. These and numerous other Christmas-market towns in the immediate area are well connected to the rail system. Have a look at the rail system just in Frankfurt's immediate neighborhood:

https://www.rmv.de/c/fileadmin/documents/PDFs/_RMV_DE/Linien_und_Netze/Liniennetzplaene/RMV_RegionalerSchienennetzplan.pdf

Are you interested in visiting other sights or just going to the markets? My favorite market was Stuttgart

Thank you Russ for the suggestion, we will definitely keep staying in Frankfurt in mind.

Right now, our focus is on the Christmas Markets but are open to other seasonal activities which sound fun.
My friend suggested visiting Ulm for the Krampus parade, which I definitely want to see!

All suggestions accepted at this point. :o)

I've never see the Stuttgart market, but it gets a lot of praise on this forum. Near Stuttgart is Esslingen, a very handsome old-world town which also gets heaps of praise for its unique medieval market. As Stuttgart is just 2 hours from FRA airport by direct ICE train, this might be a solid destination for you. And the fact that Ulm is just 40 minutes from Stuttgart by direct ICE train provides an additional reason to consider a stay in Stuttgart, Esslingen, or elsewhere in the vicinity. Heidelberg makes a good outing from Stuttgart as well.

I was in Ulm twice recently on separate visits. Due to construction the station area is a torn up mess. But seeing what you hope to see there (which I've also missed out on) will probably outshine the mess.

we will definitely keep staying in Frankfurt in mind.

Actually, I have a fairly strong preference for Mainz over Frankfurt as a base town for outings in this general area.

Frankfurt is a great hub for seeing the markets, plus it has a pretty good market itself. They even have one out in the city forest which is fun to visit, plus one up on top of a parking garage which is lovely at sunset.

Nearby markets that are open all of Advent - Mainz, Wiesbaden, Rüdesheim, Limburg, Marburg, Heidelberg.

Markets that are only open on certain times - Seligenstadt Advents Market – 28.11-15.12, Michelstadt Christmas Market – 29.11-22.12 (closed Mon. and Tues.) Ronneburg Castle Medieval Christmas Market – 30.11-1.12, 7-8.12, 14-15.12, Hessen Park Advents Market – 30.11-1.12, Ober-Wesel Christmas Market – 30.11-1.12, Büdingen Christmas Market – 4-8.12, Idstein Christmas Market – 6-8.12, Bad Homburg - weekends only, Oberursel Christmas Market – 28.11-1.12, Eppstein Christmas Market – 14-15.12

All of these are within one hour by train.

Hotels in Frankfurt that have good locations - Motel One Römer, Flemings City Center, Hilton City Center, Moxy, Ruby, Westin.

You have some good choices presented already but let me give you two more. Consider basing out of Bamberg, which would allow you to easily reach Nuremberg, Rothenburg, Wurzburg, and Bayreuth. This allows you to hit the best of Franconia/Bayern, and you can stay in a beautiful smaller city without the expense of a place like Nuremberg (which is already mostly booked). Another place to consider would be Kaiserslautern. The market there isn't very good, but you'd have access to Trier and the Mosel River cities, which are small and quaint. Also, Luxembourg, Metz, Neustadt, Speyer, Heidelberg, Stuttgart, and Strasbourg; all easily reached via the train. A plus for Kaiserslautern is that, due to its large American military presence, it's easy to find an Airbnb, or TDLA lodging for 1-2 weeks.

I haven't been yet, but near Munich is Regensburg with their Christmas market at Thurn und Taxis Palace. It is frequently rated about and we are doing a day trip there from Munich this year.

I would also say stay closer to Frankfurt--Mainz, Heidelberg, Strasbourg, Stuttgart, and in between give you more than enough. Munich was lovely, but I found Strasbourg so much better.

For several years (since 2015), we've flown into FRA (Frankfurt) and then hopped the train to Munich and based out of Munich for all our adventures until 1-2 days prior to departure, when we headed back to FRA (stayed at hotels inside the airport structure - much easier). We're departing on Dec. 10th for a 10-day trip, but this time we're flying non-stop to Munich. To answer your main question regarding which markets, here are my favorites: Munich; Stuttgart; Salzburg (go to the "egg store" (Christmas in Salzburg)!); Ulm; Nuremburg; Dresden; Leipzig. In basing out of Munich, we visited not only all the markets in Munich (S/U Bahn), but the following (via train; travel time in parens)): Salzburg (2.0) , Innsbruck (2.0), Regensburg (2.0), Oberammergau (1.5), Garmisch-Partenkirchen (1.5), Ulm (1.15), Augsburg (.30), Nuremburg (1.0), Stuttgart (2.0), Baden-Baden (3.5), Rothenburg (3.0), gone to the Zugspitze (1.5), and lots of little areas in between. From Frankfurt we gone to Aachen (2.5), Berlin (4.5), Leipzig (3.5), Dresden (5.0), Cologne (1.0), Heidelberg (1.0), and Wurzburg (1.5). We've generally split the country in two and chosen the markets from either Frankfurt or Munich. We often hit multiple markets in one day; takes a smidge of planning (go to the farthest point and work our way backwards, always cognizant of "last train for the evening" scenario. Having a rail pass ("Twin" German Rail Pass) makes life much easier - hop any train going anywhere. I've purchased the Rick Steves' Germany/Austria/Switzerland map and used a ruler and Sharpie to mark the point-to-point rail times. Makes decision making quick and easy.