I've visited Berlin, Budapest, Paris and Ljubljana.
Order of preference for revisiting -
Budapest (lots of lovely architecture, eating out, shopping, spas, jewish quarter and you can visit Buda and Pest offering different experiences, beautiful near river at night with everything lit up)
Ljublijana (lots to see in a smaller area)
Berlin (Brandenburg Gate, Reichstag Building a must - book in advance. We stayed north of river - Hachescher Markt and that was perfect for us.
Paris has a lot to offer, but I'd rather try somewhere new.
Our 20 year old DD is presently living in Germany. She's visited a few European cities - Budapest, Amsterdam, Paris, Nice, Monaco Munich, Hamburg, but says Berlin is her favourite.