It's a tough call. I'd go to different places for different reasons. I have a certain fascination with Russia, so a city like Moscow or St. Petersburg would be interesting. I'm quite a fan of Russian literature, so any number of stories take place in those cities, never mind that the authors walked the streets.
Living in the States, it's almost bizzare to consider that there are buildings and structures in Europe and Asia that are getting close to a thousand years old (if not already that old). That kind of object history has been really interesting to me, so I'd love to visit really old cities.
Living in New York City, I'd also just like to visit generally peaceful areas, like the English countryside or a rural farm in Wyoming.