The best city in the world in which to live depends on what one wants. The best city for one person would not necessarily be the best city for another person.
There is also something to be said for political and economic stability. Although we do have our problems here in the U.S., our country is one of the most stable in the world.
If one likes good weather, one could argue that San Diego is the best city in the U.S. I lived there from 1978 to 2004 and the only reason I didn't return there after living in Fiji from 1994 to 2004 is that housing prices became outrageous. But for people who can afford it, surely it is one of the best places in the U.S, although that could change because of the economic problems being faced by California.
Some surveys rate Sydney very high. I've been there several times and would not want to live there. It's far too big and getting around is too time consuming.
Surveys ranking cities make interesting reading, but I would not recommend making decisions based on them.