St. Moritz - Chur - Zürich HB is the fastest and most scenic route : hourly trains with a straightforward 6 min cross-platform connection in Chur.

You could make a detour to include a big part of the Glacier Express route but as said it's a detour : St. Moritz - Reichenau-Tamins (or Chur) - Disentis/Mustér - Andermatt - Göschenen - Zürich HB (every other hour with an easy extra change in Arth-Goldau).

St.Moritz—Reichenau-Tamins—-Disentis—-Andermatt—-Göschenen—-Zürich

Between St.Moritz and Reichenau-Tamins the Albulaline!

The fastest way between St.Moritz and Zürich is scenic too but my tip will be much more scenic.