Euston isn't badly maintained at all. It is functional and that is where it begins and ends though.So many to pick, but I’ll stick my neck out. I’m including London too.
The best maintained large station would be Newcastle Central
The best maintained small station would be Knaresborough
I’ll go one further:
The worst maintained large station would be London Euston
The worst maintained small station would be Ardwick
Wellington (Salop) is a decent traditional old station.
You may well be correct. Perhaps most rather than all though.Regarding Kings Cross, I would say all of the original was left but extensions have been added.