Like others, in no order whatsover...Mine has to be the:
West Somerset Railway
Gloucestershire Warwickshire Railway
South Devon Railway
Swanage Railway
All for a combination of the length, scenery, staffing and traction - West Somerset is a fantastic line, both in terms of scenery and length (And personally, the only downside with it is the standard preserved speed limit!)...
GWR Is another fantastic line, having to battle through landslip after landslip, but still with determination to continue upwards and onwards to Broadway / Honneybourne...
South Devon Railway - A Line that proves to me, that length isnt always best - it's a lovely line, fairly scenic, good food (Especially the Chilli!
) and a lovely variety of traction...
Swanage - Again, one of my favourites...It's a lovely line and for anyone who hasn't been there, is deffinatly worthy of a vist - everythings well looked after, the diesel gala is brilliant, foods good aswell, and come on, your going down to Swanage - What's not more to like for a Southern route