Obviously, I like the cities the most where I know people. The more, the better. But based on somewhat elusive objective standards, I'd say Zurich. This city has insanely high living standard, even for the middle class. With a populace of under 400k, it offers everything that any million-city offers, and more. Safe, relaxed, understated, clean, metropolitan, beautiful and rich. It's also the best city for finding a well-paid job. Plus, you get paid more for working less compared to neighboring countries. Some random examples: free drinking faucets every few hundred meters. Soccer goals are never locked because nobody steals them anyway (why steal, if you can buy...). If you lose your watch in the river, call the police and they'll send their diving section on a little practice session and get your watch. And so on. Some downsides: you have to speak German, most Swiss people are all a bit too nice (not very exciting people) and generally speaking, everything is almost too good to be true, so it isn't as thrilling as some other places (yes, this is a luxury problem).
Personally, I prefer Shanghai or Beijing because they're more exciting. Chicago, New York and Paris were also great cities where I could live for a while. Never been to London for more than a day, but I heard it's great, too.