Another vote for Birmingham New Street - with the qualification that, do the Brummies actually deserve any better? I mean, they proudly turned their city into Motorcar City in the 1960s, so fuck 'em.
Oxford certainly wins the vote for most congested/tiny in proportion to its significance (the Oxford/London commuters, the Virgin Cross Country Penzance to Glasgow line). Am I imagining it, or did Private Eye have a load of stuff in the early 90s on how the land next to it was reserved to turn Oxford into a proper big station befitting the university town's importance and rail connections - but with privatisation they flogged that off to the Saudi arms dealer who built his poxy business college on it?
Dundee's is ok - quite unique for it's early 80s plastic and mirrors office-stylee look.
Bradford Interchange as runner up, perhaps? Massive Victorian shed, demolished in favour of an 60s bus and train interchange, now woefully underused. Not as dirty and dangerous as it was in the 80s, though. Someone at my school was stabbed to death in there, waiting for a bus!
Or even Bradford Forster Square - another lovely big Victorian terminus, demolished, built over with out of town JBSports Warehouses, and now a dinky lego-town bus stop style station and platform....