Castle Caereinion on the Welshpool and Llanfair is a lovely quiet spot, albeit with limited facilities.
Parkend on the Dean Forest Railway is in a lovely setting, as is nearby Perrygrove.
All the Great Central stations are good, especially Quorn.
Agree with previous posters re Arley/ Highley, Tan-y-bwlch, Staverton, Weybourne, Keighley.
I enjoy a sit at Abergynolwyn with tea and cake next the one of the Talyllyn locos.
Haven Street and Horsted Keynes have atmosphere.
Both termini on Groudle Glen are good, Llen Coan in the woods, and Sea Lion Rocks on the cliff top.
Yes, Kemsley Down is odd, and malodourous....enjoyed it!
Flagging a Deltic down at Newtondale Halt is a touch bizarre......