Tough question. Most beautiful, I really like Antigua, Guatemala and Grenada, Nicaragua. Beautiful Spanish colonial architecture. Baku, Azerbaijan has some really nice 19th century Russian architecture and the old city is surrounded by city walls and has some classic old Islamic buildings. It's a bit over-restored and the city walls look more like an Indiana Jones style set but further inside it's got the typical narrow streets that end at dead ends and worn looking buildings.
Istanbul is really beautiful in the city center but it's a huge city with lots of boring suburbs. Most Turkish cities are full of concrete boxes and not very attractive. Where I'm at now, Gaziantep, is like that but in the city center it's more interesting and there's an old Byzantine era fort and some old mosques.
Tbilisi, Georgia has a really interesting city center. The buildings are old and some are literally half falling apart but it's fascinating. It might be the most interesting city that I've visited actually.
Florence, Italy may be the most beautiful city I've visited though, or maybe Sienna. Sienna is like a preserved city from the Middle Ages.
The ugliest city I've visited, maybe Qiqihaer in China. I didn't find many redeeming qualities there. Harbin, China has a Russian style city center but I found most of the city ugly and dirty as well.
Come to think of it, New York City has lots of really great Victorian era buildings in many neighborhoods. I really liked coming back to New York after 2 years in China where so many identical rows of apartment buildings is the norm.
Beijing has some interesting old neighborhoods in the city center but a lot of China is rows of identical apartment complexes, at least from what I've seen.