It is a matter of taste, as you say.
CP route through the Rockies has the edge over the CN route.
I prefer the scenery of SP's Shasta Route over the Donner (Overland) Route. I wish I had the opportunity to see the Siskiyou route from the train to compare!
BCR and WP&Y lines are spectacular!
I never thought Santa Fe had any routes in the top tier scenery list. Nice trains and service, but scenery was just nice not spectacular.
Crossing the Great Salt Lake was always a thrill.
Arkansas and Eastern Oklahoma have (had) some underrated scenic lines: KCS, Frisco, and RI.
I won't comment on lines east of the Mississippi River except to say the L&N line along the Gulf Coast appealed to me.

Wish I could have seen from a train: Copper River & Northwestern, SP line to Cloudcroft (NM),
SPC line to Santa Cruz from Los Gatos, Burlington's Wind River Canyon line, Milwaukee Road's Western Extension, and finally NP's mainlines in Montana.