On a Northern 158, in order of preference:
Best is an airline-style seat, backwards-facing, next to a window, in a driving (not middle) carriage. Preferably not the ones with restricted legroom due to the heater under the seat in front - but those are better than anything below. The armrests next to the body can't be raised in the centre carriages, so these are less comfortable, but again - better than anything below.
An exception was during *exceptionally* hot or cold weather, because the air conditioning in the centre carriages did occasionally work, and their heaters are also more effective in general. I'm sceptical that any of the a/c will still be working at all by next summer so this is probably no longer a consideration.
Next is forward-facing in an airline seat next to the window.
Then we're looking at an airline seat, corridor side. The best choice is always one where some inconsiderate person has tried to "reserve" an empty seat with their bag - failing that, then next to someone thin.
Then if there are no airline seats at all, and if it won't get in anyone's way, the luggage rack is the best option. The wide one for preference, the slim one in a pinch.
Then, if there's too much luggage or too many people for a luggage rack, and again if it won't get in anyone's way, the tip-up seats in the ends.
Then, if there's no other option, a table seat.
Then we're in to "sitting on the floor" territory, and the best spot is by the wheelchair space, and if that's full then in the very centre of the carriage where people are less likely to need to get past.
It is, in general, better to be towards the middle of the carriage than the ends. Less through-traffic of people, less draught from the doors in winter, slightly less pitiful ventilation from the open windows in summer.
Yes I probably have thought about it too much.