I didn't visit all Europe (i visited so little actually), still:
By living standads (fun, crowds, things to do):
Constantinople: this is simply the best in the world.
In Italy I'd say Rome is the funnier.
London and UK was a huge disappointment (after midnight nobody is around).
Honorable mention:
Sevilla
By beauty (geographical setting):
1- Constantinople: again, it's unique, 2 continents, 2 seas, 2 water bodies, and the hills and forest and islands.
Honorable mentions:
Venice: well, it's just in the water.
Rome: seven hills, lots of panoramic view
Naples: Vesuvio in the background, sea in the front
San Gimignano: amazing, at the top of an hill in the tuscanian country.
Assisi: top of an higher hill
By beauty (historical arts and buildings):
1- Constantinople (again
): well, if you love byzantine art (like I do), it's simply overwhelming. Ottoman, ancient, modern and even italian arts are standouts.
Honorable mentions:
Florence: my hometown! Art at pure state.
Venice: unique (a mix of byzantine and italian art)
Rome: if you love roman art as well as baroque art, is the place to go.
Granada: Alhambra, a dream.