If I had to pick one for atmosphere & scenic splendour, my choice would be the Callander & Oban, including side trips on the branches to Killin and Ballachulish.
However for personal and local interest (and much less glamorous), I would have liked to have travelled on all of the extensive network of lines which criss-crossed south Lancashire and northern Cheshire up until the Beeching era.
Many of these were not "branch lines" in the traditional rural sense, rather alternative or duplicate secondary lines connecting larger settlements which are often still on the rail network today, but were obviously an easy target for Beeching's axe
I have a modest collection of old timetables and sometimes while away a rainy Sunday afternoon using these to work out track bashing trips covering long-closed lines in my local area.
I'm not sure how long Ranger and Rover tickets have been around (weren't they called Runabout tickets back in the day?), but here's one typical example when we set the Time Machine's dial to a weekday in April 1950, pull on the gabardine mackintosh & trilby hat and set out for the day with a pocket full of sixpences, shillings and half-crowns, or a hypothetical Lancashire Runabout ticket valid from 08.45 onwards.....
Positioning move is a Ribble bus from Wigan to Blackburn, then:-
08.46 Blackburn - Padiham - Rose Grove arr 09.12
09.28 Rose Grove - Manchester Vic arr 10.21 (running via Ramsbottom, Bury & Clifton Jn, stopping only at Accrington and Salford)
11.00 Manchester Exchange - Tyldesley - Wigan North Western arr 11.51
12.53 Wigan NW - Red Rock - Chorley - Brinscall - Blackburn arr 13.49
14.03 Blackburn - Preston arr 14.27 (via Todd Lane Junction - none of this Farington Curve malarkey)
15.02 Preston - Hesketh Bank - Southport Chapel St arr 15.46
16.15 Southport Chapel St - Burscough Junction - Ormskirk arr 16.42
17.06 Ormskirk - Skelmersdale - Rainford Junction arr 17.24
17.35 Rainford Junction - St Helens Shaw St arr 17.51
17.56 St Helens Shaw St - Widnes - Ditton Junction arr 18.26
18.48 Ditton Junction - Warrington Arpley - Manchester Oxford Rd arr 20.19
20.30 Manchester Central - Wigan Central arr 21.29
If the dash from Oxford Rd to Central didn't work out, it would be a wait for the 21.40 Manchester Central to Warrington Central, changing at Irlam for the 22.16 to Wigan Central and arriving at 22.52 just in time for the last bus home from the town centre. Or if an earlier night is needed ahead of tackling the Southport Lord St and Glazebrook to Tiviot Dale lines tomorrow, options were available from Exchange to Wigan NW via Tyldesley or Victoria to Wallgate via Atherton.