The best cities to live on Earth

Better weather, schools, less taxes and more culture. Taking your life and moving it overseas is something most of us have daydreamed about at some point.
The question is where should you head to? Fortunately for those wondering if the grass really is greener, companies around the world conduct detailed studies on where has the best (and worst) quality of life, so they can work put how much they need to compensate employees for sending them abroad.
Consulting, outsourcing and investment company Mercer's 2010 report has taken a look at 221 of the world's biggest cities and ranked them according to 39 criteria, including political and socio-economic factors, environmental factors, health and sanitation, education, transportation and other public services.
Here are their 20 best places on Earth, as well as the worst.