Have you tried trainscanbecheaper.info? According to that, you can save a couple of quid by 'splitting' at Leeds (using Off Peak Day Returns - which may, or may not, meet your needs depending on what your travel times are and if you are returning the same day) however that is not the
cheapest price.
'Best' has not been defined, but if 'best' means cheapest, then you need to 'split' (and therefore the train must call) at:
- Glazebrook
- Littleborough
- Walsden
- Micklefield
- Church Fenton
- Northallerton
Note that is not 100% definitely the cheapest, but is likely to be the cheapest. This saves around 1/3 of the price, but at a time penalty as you would need to get Northern trains to York, then TPE* between York and Darlington. This also assumes you return the same day.
(* There are also a few EC services that call at Northallerton)