MOST BEAUTIFUL CITIES IN THE WORLD

Saint-Petersburg, Russia
Lagos, Lissabone - Portugal
Guntok - India, Sikkim
Dubai - UAE

Istanbul
Paris
Prague

In my opinion, cities of the Old World have more charme, cities of the New World are easier to adopt if you are a foreigner.
When it comes to beauty, I think nothing beats Mediterranean towns and cities.
Here is my list from my travels:

Towns and small cities:

1) Venice
2) Brugge
3) Salzburg
4) Dubrovnik
5) Heidelberg
6) Oxford
7) Zurich
8) Quebec City
9) Toledo
10) Savannah

Major Cities:

1) Roma
2) Barcelona
3) Paris
4) Prague
5) Vienna
6) Florence
7) Munich
8) San Francisco
9) Buenos Aires
10) Istanbul, Sanaa, Isfahan

Have not yet been to: Dubai, Shanghai, Tokyo, Sydney, Rio, Cape Town
Very nice: Berlin, Bordeaux, Amsterdam, The Hague, Oslo, Stockholm, Copenhagen, Madrid, Krakow, St. Petersburg, Kiev, San Diego, Boston, Hamburg
Was not impressed by: Vancouver (granted, beautiful setting, but other than that?), Toronto, Houston, Dallas, Miami, Brussels, Cairo, Abidjan, Lima, La Paz, Guyaquil, Caracas, Helsinki, Moscow, Baku, Ankara, Tehran, Amman, Los Angeles, Frankfurt, Cologne (yet great people!)

La Habana, Cuba
Trinidad, Cuba
Venice, Italy
Barcelona, Spain
Buenos Aires, Argentina
San Diego, USA

also not bad...

Lima, Peru
Amsterdam, Netherlands

I think this is a seriously hard question, just so many to choose from !

I'm going to keep to three for now...
Roma
Sevilla
Salzburg

Vancouver, Canada

There are so many to choose from! I haven't been everywhere, but these would be my picks for 'most beautiful cities':

In the U.K.: Edinburgh
Canada: Victoria, B.C.
U.S.: San Fransisco
Russia: St. Petersburg
China: Suzhou or Qingdao

Also: Rome, Helsinki, Paris (some areas) and Athens at night.