In terms of something interesting to pass the time*, Blackfriars is hard to beat. The view over the Thames with its endless flow of boats against a world famous skyline never fails to grab my attention, day or night - as long as you are wrapped up well in colder times!
* It's ironic that as the service through the Thameslink core is so frequent, there's rarely a need to spend much time admiring the view at Blackfriars.
A +1 from Blackfriars here, for London stations anyway!