As a landscape, no city I’ve seen can beat Rio de Janeiro.
At a closer scale, however, I would go for Paris.
Sydney, Australia
Also San Francisco
Several of my top picks have already been named, but here’s one that hasn’t been: Bern, Switzerland. The central, old part of the city is a UNESCO World Heritage site because the medieval layout and design has largely been preserved. There are lots of trees and hills, and the Aar River loops around the Old City.
The Wikipedia page has several nice photos.
Of the places I’ve been, I think this might top it.
Runners up (in no particular order):
Prague
Lucerne
Edinburgh
Freiburg
Dresden
Salzburg
Dubrovnik
Krakow
I lived in Budapest, and I do consider it quite beautiful in its own way, but also quite rough around the edges. I was born and raised (and currently live) in Chicago, and I would consider it beautiful, too, but I’m biased. Seattle would be my other standout in the US. The day I spent in San Francisco also left me with a favorable impression.
Oh, one place I think is so beautiful that I fantasize about retiring there someday is Ohrid, Macedonia.
The fact that it’s also pretty damned obscure (outside of the Balkans, where it is a very popular vacation spot) is just a bonus. 
Personally, I’m sick of trees, mountains, oceans, sunlight, nightclubs, casinos, etc.
I’m a people person, and I like to see (preferably female) people partying and having fun.
Rumor has it that the most beautiful people are in Los Angeles, New York, and Paris.
Istanbul, Prague, Budapest, Istanbul, Tallinn, Istanbul and Istanbul.
Milton Keynes.
I’ve never been there, but looking at books on its architecture, I’d have to say St. Petersburg deserves a place on this list.
Trieste, Italy. The buildings around its main piazza (right on the sea) look like wedding cakes.
Is it just me who gets a kick out of the fact that the Fairmont Le Chateau Frontenac is featured in all of those pictures? I guess it really is the most photographed hotel in the world.
Prague and Barcelona top my list too. Paris is very much like London to me - lovely in parts, but with too many run-down, ugly or boring parts to compete with Barcelona or Prague.
Seattle, Jackson Hole Wyoming, Taos New Mexico, SanFrancisco, Reykjavik.
nice to see all the Cape Town love. Unexpected (but true!)
Other than Cape Town, the most beautiful city I’ve seen is Venice.
Another vote for Prague.
I find baroque aritechture and quaint European streets ungodly boring. It’s all about the exotic mystery, to me. The above-mentioned Jaisalmer is lovely, My other favorites are:
Kathmandu (easily my favorite city in the world, ever)
Djenne
Lijiang (crowded and has a Disneyland aspect, but postcard perfect)
Lhasa (though the new stuff is appalling)
Yangshou
I’ve traveled enough to get this joke.
My vote is Paris. I absolutely fell in love with the architecture. I have found others places exciting or interesting, and thought many places were picturesque, but I have never seen another city that I thought was so beautiful.
Gloomy grey is the natural weather of Paris, and that’s how the city should be discovered and appreciated. It’s beautiful and gloomy, bit melancholic.
I love autumns and winters in Paris.
For me , it’s either Paris or Rome.
Listed from my personal experience:
Prague
Istanbul
Sienna
Paris
Florence
Rome
Vienna
Obidos (Portugal)
Lisbon
Seville
Salzburg
Bern
Shhhh was tryna get the kid to visit Milton Keynes.
I’d always thought that my answer to this question would be “none,” but the centre of Paris is easily the most beautiful city on Earth. That’s the rub, though, because Paris isn’t just its center core; it’s also all of the same ugliness built up around it that disqualifies any other city from my consideration.
If we can only talk about certain parts of cities, then Detroit is quite beautiful, as is Chicago, Mexico City, Ghent, San Antonio, Brugges, Cologne, Frankfurt, Mannheim, Tours, New Orleans, Toronto, and hundreds of other places. But if you want to take things on the whole, I’m ready to even drop Paris from my list.