A couple more of mine:
Mole Valley Line - Leatherhead to Horsham
A lovely little line with wonderful views of the North Downs for the whole journey once you are out of London. I did Horsham to Leatherhead in summer in glorious sunshine and it was wonderful, like being abroad, super views of distant hills from an air-conditioned 377 (I am from East Anglia so am used to 321s :P).
Arun Valley Line - Horsham to Ford
Another wonderful line from Horsham, this time running through the South Downs to the coast. A very nice line with wonderful views of wooded hills and Arundel castle, through charming little Sussex towns like Pulborough and Arundel.
Berks & Hants Line - London Paddington to Taunton
Most people think of Exeter to Penzance of being the nicest part of the journey west, but I love all of it. From views of the Thames in Maidenhead to and rolling hills and white horses of the Vale of Pewsey, to the lonely spooky stretch of track devoid of stations between Castle Cary and the junction with the Cross Country route through wild lands, with views of distant hills, I love this line. Also, it is done on FGW's HSTs, IMO the best trains in the country.
Folkestone Central to Dover Priory
Only a short journey, but with the superb view of Folkestone town from the viaduct to the coastal stretch through Samphire Hoe and being able to see France on a clear day, this is certainly worth doing.
Ashford to Canterbury West
People think of the Marshlink line to Hastings via Rye as the only scenic line to go to Ashford but this is a very nice, very pleasant journey through the Kent Downs. Also "The Tickled Trout" pub in Wye is well worth a visit.
Chatham to Swanley
I was visiting Greenwich Uni's campus in Chatham for a look round and was disappointed with the whole journey down from Cambridge. CBG to KGX is dull tbh, and KGX was a building site at the time. HS1 is very dreary as far as ebbsfleet, with a brief view of the QE2 bridge the only highlight. I took a stopping train to Victoria back and the countryside bit of the journey was wonderful, with good views of the Medway viaduct and the lovely little oasthouses that litter the North Kent Downs.
West Anglia Line - London Liverpool Street to Cambridge
I did this on the same day as the previous line. I was looking for a better route to CBG than the ECML and I deffo found one. The Lea Valley was surprisingly nice, through lovely towns like Sawbridgeworth and Stansted Mountfitchet to the weirdly bleak and empty bit through Great Chesterford that is fascinating IMO.
The Brecklands & Fenland Line - Norwich to Peterborough
From the massive wilderness of Thetford forest to the eerie flatness of the fens this line truly has it all. The stretch through Shippea Hill with the black soil is reminiscent of a desert IMO.The view of Ely marina and Cathedral is brilliant And it serves so many ghost stations (Spooner Row, Eccles Road, Harling Road, Lakenheath, Shippea Hill, Manea) that it boggles the mind. The massive King's Dyke brickworks and wind turbines at Whittlesea(y) are fascinating on an industrial level also.
The Highland Main Line from Perth to Inverness would be my candidate. Has some interesting engineering (Culloden viaduct) and a lot of spectacular scenery, including the well-known purple glens. If it weren't for the West Highland Line it would probably be talked about a lot more.
Good example, I always thought the HML would be nice, but I have never heard anything about it.
Horsham-Ford and Parson St-Taunton.
A fellow Arun Valley fan eh :P
I do like the Marshlink (Ashford to Hastings) line. Whilst the areas at each end of the line are built up, the line between the two goes through what must be some of the least populated parts of Kent and East Sussex. While you would imagine it would all be flat, there are some hills to the west of the line around Rye. On a sunny day it's beautiful but it's definitely one of those lines which feels very bleak if it's dark and rainy.
I was very impressed with the Arun Valley line when I travelled on it the other week. Arundel Castle is a major landmark on the west of the line and it's scenic pretty much all of the way from the South Coast to Horsham.
The Breckland line from Norwich to Ely/Cambridge is interesting and picturesque too. In some parts you are almost surrounded by forests and as you approach Ely you see the dark Fenland soil in the surrounding fields.
You have excellent taste :P
I am a big fan of all of those lines, the only reason I didn't put the Marshlink line was that I have it down as quite a well known scenic line.
Another couple from me - Faversham-Canterbury East and Maidstone Barracks-Tonbridge.
I haven't done either of these, what are they like?