First I should give an overview where I have been, because I haven't yet seen some major european cities like Paris or Madrid.
Out of those: Hamburg (my hometown), Berlin, Hanover, Kiel, Bremen, Enschede, Amsterdam, Rotterdam, London, Copenhagen, Warsaw, Prague, Zagreb, Ljubljana, Venice, Padova, Palermo, Istanbul, Valencia, Barcelona
I would put those cities into my top 3 (in order):
1. Ljubljana
2. Hamburg/ Barcelona
3. Valencia/ Prague/ Berlin
Barcelona!!of course!! For my is the best in Europe and in the world!! I think is the city that gives you the best offer in services, culture and leisure time.
others: Paris maybe the most romantic and London maybe the most cosmopolitan.