Berlin
Berlin offers a wonderful combination of history, night life, architecture and culture. There is no city in Europe that parties harder than Berlin. The nightlife of most cities is tame in comparison to this city. There are lots of great clubs and dance parlors. Finding a good place to live should also be no problem.
I usually go to Berlin for conferences. If you want some tips, here they are: find a hotel in the city center, called here Mitte. You will have almost everything within walking distance: cinemas, parks, pubs, discos etc.
I am also a big fan of rollerskating and Berlin is one of the best places for urban ride. If you have skates, take them with you. City tours are much nicer on skates at night.
Berlin although not as tremendous city as Paris, has a lot to offer. A great choice of restaurants, shopping centers, hotels (both expensive and cheap) and a lot more!