I was impressed by the Ourense-Zamora section of the Galicia-Madrid line, high in the mountains near the Portuguese border.
It's rather like the Settle & Carlisle, but goes on for hours with countless short tunnels as it switches around the high peaks.
Run (on diesel on this stretch) by Alvia S-730 bi-mode/bi-gauge trains.
But it's on borrowed time, the new LAV is being built nearby, but mostly in tunnel at a lower level.
Anywhere on the RhB of course, and across the Alps generally.
Prague-Dresden and all 3 routes from Zidani Most in Slovenia have lovely river/gorge scenery.
Also the Ardennes lines in Belgium, and many lines in central Germany (eg around Fulda), for ever-changing hilly country.
The whole Mediterranean coast from Genoa to St Raphael is great, except where FS/SNCF have recently put the line "round the back" of the coastal towns instead of on the sea front, or even underground.