Just to add my thoughts :
For a major (ie: significant terminus station), personally I don't think it gets much worse than Euston. What a shame too, those shiny new trains and all the glitz of being on a virgin train at that, and you get a nasty, dark, dirty concrete block of a station.
At the other end of the line comes Birmingham New Street in close second, though I will admit the redevelopment in recent years is a great improvement on where it was. It's pleasing enough until you reach the platforms!
For more moderate sized stations, worthy mentions have to go to Manchester Victoria and the fume filled crypts that are platforms 3-6 (though the concourse and p1&2 are lovely). Would also lump Wakefield Kirkgate in this category, better than it was but still a long way to go (and what a stark contrast to Westgate!) Despite mentions earlier in the thread, I personally find neither Bradford too offensive. They're not overwhelmingly pleasant, but are perfectly acceptable as far as function is concerned.
Dundee, for me, also gets a mention. Despite the town (City?) being rather pleasant the station itself manages to give a rather depressing welcome.
Thats all I can think of for now!