The Economist Intelligence Unit :

Although the most liveable cities in the world remain largely unchanged, there has been movement within the top tier of liveability. Of the 65 cities with scores of 80 or more, six have seen a change in score in the past 12 months. While most cities in the top tier have registered an improvement in their scores, two of them, Manchester in the UK and Stockholm in Sweden, have seen their scores decline as a result of recent, high-profile terrorist attacks.

Over the past few years several US cities have registered declines in their scores. This stems in part from unrest related to a number of deaths of black people at the hands of police officers. In addition, the country has seen protests held in response to President Trump’s policies and executive orders.
Sydney in Australia is another city that has seen a decline in its ranking, reflecting growing concerns over possible terror attacks in the past three years. Sydney now ranks outside the top ten most liveable cities, at number 11, down from seventh place just over a year ago. Nevertheless, with such high scores
already in place, the impact of these declines has not been enough to push any city into a lower tier of liveability. Although 17.2 percentage points separate Melbourne in first place from Warsaw in 65th place, all cities in this tier can lay claim to being on an equal footing in terms of presenting few, if any, challenges to residents’ lifestyles.