Settle & Carlisle is always up there. As are Exeter - Newton Abbot, Edinburgh - Aberdeen ( I think this line is the most underrated), the highland mainline, far north line and Kyle of Lochalsh line.
The Heart of Wales line is nice, as is the Cambrian Coast and to some extent the North Wales Coast.
The ECML from Newcastle to Edinburgh is worth a shout, as is the WCML over Shap and Beattock.
Going across the Pennines, The Calder Valley and Hope Valley lines are also scenic, indeed as is the line to Buxton.
Turning to preserved lines, the top one must be the Welsh Highland, followed by the Ffestiniog, North York Moors and Snowdon Mountain Railway (in no particular order).
Honourable mentions must go to the line across the King Edward Bridge over the River Tyne and the lines crossing the Thames in central London if you find urban landscapes scenic (there will be others too)
It depends what type of scenery you are into I suppose, and it is all very subjective. As they say, beauty is in the eye of the beholder!