Hamburg Hbf, good facilities and lots of memories associated to it from my year in Germany, though a bit "rough" in terms of people hanging around it as European main stations often are.
Or if you like architecturally interesting - how about the "Hundertwasser-Bahnhof" at Uelzen (lots of pics if you Google it)? A small and fairly insignificant regional station re-imagined by one of the quirkier architects Europe has to offer.
I agree with Milano Centrale's architecture - a veritable temple of transport. And Berlin Hbf - just impressive (and good facilities too, you can certainly get a decent Currywurst!
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Gare du Nord might be interesting, but it's just a dump. Flatten and start again would be the best plan, including much of the surrounding area. Same with Brussel-Zuid.
Possibly Montreux from holidays in Switzerland - the few minutes each side to Lausanne and Villeneuve are among the best standard-gauge journeys anywhere.
The Montreux Riviera line (if you'd call it that) is probably my favourite mainline journey anywhere in the world, and I was highly privileged to have it as my weekly commute for a couple of years around 2011, and much as I don't miss getting up at 3:30am every Monday to get there I do really miss the place. Interesting and scenic in different ways all the way from Geneve-Aeroport to Brig. But the stations themselves are generally fairly basic, though functional, and the facilities at them aren't great.
In terms of Swiss stations I'd probably go for Bern Hbf. It's a bit of a New St but done well, and of course no diesel to fug it up with fumes, because the Swiss have a bit of sense in that regard.
