My favorites would have to be NY or Istanbul. Istanbul is an amazing city with incredible history. You could spend weeks visiting historic sites there. It has a really lively nightlife, I'm not into clubs but they have bars where you can sit outdoors in the alleyways with live music sometimes. It's neat having a city on two continents too. As far as living there, it can be a pain. It's a huge city and sometimes you find yourself having to take the bus, the ferry, then the metro as you have to make it across two continents.
New York is a great city and has convenient transport which is a big plus. It's perhaps the most diverse city in the world, I'm not one of those people who constantly has to be surrounded by a diverse crowd to feel culturally relevant or whatever, but you can certainly find food from every place which is a big advantage. It's not always so expensive either, me and a friend stuffed ourselves with sushi for just a little over $20. When I first visited I didn't like it that much but now I can see myself possibly living there. You can kind of get used to a neighborhood and forget you're living in a huge city. There are lots of old neighborhoods with nice Victorian architecture so you don't feel like you're living in rows of identical buildings.
I like Beijing, it's got some really interesting historic sites and interesting old neighborhoods. It's not exactly my favorite city though.
