The best city in the world is Helsinki, followed closely by Lisbon. At least that's the conclusion of users of CityClash.
CityClash is a web app that produces rankings of the best cities in the world based on user input. To add your rankings you must choose two cities that you know well. CityClash uses Google Maps to help you in your choice (the maps include some rather nice animated clouds). Once you have picked your cities you are then asked to rate them in a number of categories.
You can view the best cities worldwide (as decided by comparing users ratings) or view the best cities in a particular country. It is also possible to select a category, such as food & drink or culture and view the best rated cities in that category.