Best scenic rail journey?

Either Bernina/Albula route (Tirano - St Moritz - Chur) or Glacier Express (Chur - Andermatt - Brig - Zermatt).

Depends on which fits in better with whatever route and stays you have planned. Unless you are heading south-north between Milan/Lugano and eastern or central Switzerland the Bernina is in the wrong place. And the Glacier Express is only good if you are crossing Switzerland East-West. Do not get hung up about doing every Km of the Glacier Express route, the best bits are in the middle, so skipping the ends is not a big problem.
There are lost of other good routes as well. Design which routes you take around where you want to go, not vice-versa.

We loved the Bernina Express last summer. We built our whole trip around it, adding Murren and the Berner Oberland at one end, and Lake Como, Milan and Venice at the other.

Here are some resources to guide your research:

The Man in Seat 61: https://www.seat61.com/BerninaExpress.htm

YouTube: https://www.youtube.com/results?search_query=bernina+express

I particularly like the northern half of the Bernina route, especially around Davos and the Landwasser Viaduct. Andermatt to Disentis on the Glacier.

The Brunig Pass route between Luzern and Interlaken Ost is one of my absolute favourites, via Meiringen and Brienz.

I enjoy the western half of the Golden Pass between Interlaken West and Montreux especially the centre where the language border is, especially on the school train (think school bus on rails) listening to the children speaking bilingually.

If I had to choose, probably within 15 km of the Landwasser Viaduct would do me fine.

It depends on what kind of scenery you want from your journey. My vote is for the Golden Pass Line from Luzern to Montreux!
From Luzern (Lucerne) near its lake, river, and old town it heads south past lush, vivid-green pastures and blue-green mountain lakes with chalets and their bright flower window-boxes and gardens. After it goes over the Brunig Pass, you'll see a series of waterfalls on the other side of the valley near Meiringen and head west to bright blue-green Lake Brienz. You change trains at Interlaken Ost for the second stage of the trip (or you can take a diversion on a little regional train into the Lauterbrunnen Valley for a few days to visit snow-capped mountains like the Jungfrau and the Schilthorn and hike through flower-filled pastures with stunning views, before returning to Interlaken Ost to resume your rail journey.) From Interlaken Ost you take a train to Zweisimmon, which passes the large blue Lake Thun. When you get to Zweisimmon, you change to a very special train car, for which you want reservations with an upgrade to first class for this third stage of your trip--while the same view as in second class, the first class reserved train cars are very plush on this last leg of the trip. You'll pass more lush green pastures in rolling hills and mountains with chalets and gardens, pass famous Gstaad, then go through the wine-grape-growing region dramatically down to Montreux and Lac Leman (Lake Geneva) at the end. That's near the famous and impressive Chateau de Chillon (Chillon Castle.) The whole route has large panoramic windows in both first and second class.
"My Switzerland.com" is a very helpful resource:
https://www.myswitzerland.com/en-us/goldenpass-line.html

Re the Golden Pass, remember every train car passes the same scenery. I was in first class between Interlaken and Montreux just because I had a rail pass but didn't feel the need to be there, you'll still see the great views.

There's an episode of the series Mighty Trains that is devoted to the Glacier Express, I'm sure it's on Youtube somewhere otherwise it aired on the Smithsonian Channel and they repeat stuff all the time. I also found several semi-professional Youtube videos for the Bernina Express when I was looking into it (and yes I want to do it!)