I've been studying France and french culture, so I'm a bit biased, but I would say Croatia.
Seriously though, upon looking it up, the cities are actually some of the most beautiful I've seen in a while, but nothing to beat out my interest in the good old city of Paris.
I've heard good things about Croatia as well. But some people told me Prague was one of Europe's most beautiful city (I haven't been, so I can't tell). I've been to Paris though, and it is awesome.