Barcelona! Because of the many exciting architectures and nightlife. It also has tall buildings.
London and Prague are also really good, but London is too cold and Prague has no skyscrapers. And Paris is too tourist unfriendly. Haven't been to Madrid and Milan though.
Well, from a objective view I would say Paris or London.
"My style" of cities, however, are Hamburg, Amsterdam, Copenhagen, Berlin etc.
Rough-n-rugged, liberal, party, beer, progressive architecturewise and so on.
Rotterdam have's the second biggest harbour in the world. And it has old historic buildings. But it is also the city with the most skyscrapers in the Netherlands. I love it.