My personal all-time favourite to travel on is the West Highland Line. It's been many a year since I last did the line north of Arrochar & Tarbet, granted, but the memory of the line is still strong. Especially of the Oban line when the sleeper was diverted there in February 2006
Just the revisit as far as Arrochar this October, as part of my first ever cycling holiday, was enough to remind me just how much I need to go back there
It's been many a year since my last visit to the Kyle line too, I believe that was in 2006 as well. I've just remembered the epic tour to Thurso in 2007 with 40145, now that was fun
Never drunk so much Irn-Bru in one day
The line to Kyle, I definitely need to go back there too, my memory of it is still strong of the beautiful scenery. I also have a strong memory of getting the feet extremely wet going over the Skye Bridge after walking through a deeper than expected flooded dip
Highland Main Line, certainly very nice to do but compared to the WHL or the Kyle and Far North lines, it's in a different league.
Now if we're going outside the UK, it takes some doing to beat the Bernina Express between Chur, Switzerland and Tirano, Italy
Dombas to Andalsnes in Norway is also very scenic and well worth doing.
In terms of pure fun though, racing along the GWML at 125mph while relaxing with a beverage, staring out of the window, that takes some beating.