Glacier express train in December

I like Andermatt to Disentis/Muster over the Oberalp Pass. Then Chur to St Mortitz on the Albula rail line with all the loops and the iconic Landwasser Viaduct.

I think it depends on where you are going afterwards, or coming from.

I found the entire journey from St. Moritz to Zermatt to be enjoyable. I was not bored at all. There was always something to watch out the windows. And I even went on a cloudy/rainy day. I also would make your decision based upon where you want to start and end your trip given your overall itinerary for your stay in Switzerland.

However, if you are turned off by the thought of sitting on a train for 7-8 hours, then perhaps you should pick a portion of the journey.

And if you'd rather pick just a portion, then I agree with Sam: east of Andermatt towards St Moritz is more scenic. Andermatt is easy to reach from central Switzerland (e.g. Zürich or Luzern) or from the south (Lugano or Locarno).

This is a popular subject for vloggers, perhaps watch a few and see how much you want to deal with. If I ever do it, I'm stopping overnight in the middle.