Dubrovnik.
But excepting the obvious, there are a few contenders.
Istanbul- Sultanahmet Park is the best part I've ever been to. Hagia Sophia on one side, Blue Mosque on the other, beautiful (although by no means monumental) fountain in the middle. From the Bosphorus, the city is also quite beautiful, but in a rather dirty and dusty way. So it's rather unconventionally beautiful.
Oslo- Specifically, the new bit around the Opera House. The Opera House is spectacular in itself, and the new office buildings are very elegantly designed (I especially liked the new PWC
)IIRC) one). Even the more downtrodden Grønland area is quite pretty. Frogner Park is also quite notable in its tranquility and beauty, even if the centrepiece is notoriously suspect. The beauty in Oslo is quite definitely a modern one, and again unconventional, but I like it! Also, those Norwegians really know how to do their water features, which adds to it!
Sydney- Sydney Harbour, specifically. Maybe I'm just biased, but it really is exceptionally pretty, especially on a day when the sun is shining. Even the skyscrapers give it a certain metropolitan beauty. I've heard the Opera House described as ugly, but I cannot for the life of me figure out how it is. It is highly aesthetically pleasing.
Amsterdam- So nice and quaint and pretty! The canals lined with beautiful trees.
Bern- The cathedral on the top of the hill is the highlight, looked at from the opposite side of the river, from around about the bear enclosure.
Timisoara- The centre is surprisingly modern and clean, and the Metropolitan Cathedral is without a doubt brilliantly beautiful. There are also heaps of gardens and parks, which are for the most part well kept and manicured.
And a couple of the ugliest:
Cologne- Not that it's
exceptionally ugly. It's just not pretty at all. You argue that the cathedral is nice, but it is blackened and dirty, and is situated next to the train station, whose bridge across the Rhine is not exactly nice!
Bucharest- I got out of here as soon as I could, but what I saw was horribly downtrodden and in need of urgent repair. It's pretty notorious for being ugly, so it seems a safe bet.
Belgrade- I think they've made an effort to make it look at least semi-respectable, but it's by no means pretty. It's still largely a stereotypical Eastern European city.
Edit: Bah! I forgot so many pretty ones, particularly Lucerne, Innsbruck, Salzburg, Tallinn, Brasov and Split.