My fave European city is London. I've never been, however, but it's my fave simply because it is the center of culture and refinement for pretty much the entire world. Nowhere on earth could ever top it. Not to mention it is the heart of our mother country.
Regarding the whole "city" thing, it's really simple. 500+ people and you get to be a city. 499- and you're a dirty stinking village. Markets, cathedrals, and so forth should play no part in it.
I have lived in a building with 500+ people.*
* I think. Didn't count them.