My favourite is the Settle and Carlisle line, a magnificent route. A close second is the Peak Line from Chinley to Ambergate, I have only travelled on the now open parts, but I have walked the rest. Both ex Midland lines, they knew how to build a decent railway.
the only line that could compete with those two is the Somerset and Dorset line from Bath to Bournemouth, never traveled on it, but seen many pictures and films of it. I hope that the work of the New Somerset & Dorset Railway people manage to reopen it before I die. I have already travele don the short bits south of Midsomer Norton, cannot wait for the extension.