There really is no best city that suits everyone; it really depends on your interests. However, in my experience, it would be wrong no matter where you choose to go. Copenhagen and Amsterdam are wonderful, but so are Prague, Berlin, Stockholm, Rome, Florence, Venice, Budapest, Vienna, Barcelona, Geneva, Zurich, Paris, Bruges, Munich ... I've been to all of them and more and would be glad to go back to any of them. In your case, Amsterdam is close and involves relatively little travel time. There are hundreds of museums covering about every conceivable topic, historic sites like the Anne Frank House, canal trips, festivals depending on exactly when you go, and lots of restaurants and shopping.