As I see it, as far as travel in Europe goes, you have the big three: London, Paris and Berlin (some people feel that Rome should replace Berlin on this list, but they are wrong). These are the major hubs – centers of culture, history, and tourism. So you if you wanted to get a good feel of Europe with the most stuff to do, you’d want to make sure you hit those cities.
But you should also note that some of the slightly smaller European cities provide equally amazing experiences, without the huge crowds. Some of those cities would be Florence, Amsterdam, Budapest and Vienna. And for me, Prague is on the top of that list.