For London, I would give it to London Bridge and Waterloo, for the following reasons:
London Bridge as the station was completely modernised in a manner to make it more easier to use, and increase capacity, yet unlike "Euston" it was done in a architecturally impressive manner, and a good example of modern design. Waterloo because in my humble opinion, it was built at possibly the peak of classical railway building, and maintains a lot of its old features, and modern additions such as the International section, were marvels in their own way. Kings cross is extremely close runner up to Waterloo. St Pancras looses a bit because of the modern addition is a bit uninspiring.
Outside london, I give both Reading (similar reasons to London Bridge) and Britsol Temple Meads.