In the US, Philadelphia 30th Street is the hands-down winner. It has it all; huge capacity, splendid architecture, massive office space, and authenticity. To satisfy the bean-counters, the original office towers have been joined by a big new one over the north tracks which were previously open. Train tracks on two levels avoid bottlenecks, E-W and airport SEPTA upstairs; N-S SEPTA, Atlantic City and Amtrak downstairs. There seems to be plenty of spare capacity. The spacious concourse means passengers changing trains aren't pushing past each other to get around, as at Penn or other 1960s subway caricatures around the world (e.g. Birmingham New Street in England).

Another vote for Newcastle Central; except with the caveat that it is mostly a long-distance station (the vast majority of trains go to destinations anywhere from 100 to 300 miles away) with very few commuter services. So it probably isn't a good blueprint for New York Penn where Amtrak is only a small part of the traffic. Fabulous architecture, nonetheless.

I don't think other continents are going to provide much of an example, unless Japan has some material to work with. Australia, Latin America and Africa are for the most part too low in volume to be relevant and I'm not sure if New Yorkers really want China or India as a blueprint.