What qualifies a city to be considered the best to live in? If you were to set your own criteria, would Vancouver still be the best city in the world for you? I know of people who simply cannot adapt to the cold weather; such a person would not put Vancouver at the top of the list.
Melbourne is rated second or third, depending on which list you look at. But it won't be, if you are prone to hay fever. It is probably the top of the list for highest number of people afflicted with severe hay fever during the pollen season.
If you have a small amount of savings in your retirement, would you live in Sydney, Vienna, or Toronto? Perhaps you would rather to go to a place where the cost of living is much lower. A modest amount of savings in some third world countries would go much further than in one of the listed "most liveable" cities. Thanks to Julia Gillard, Australia now has a self-imposed carbon emission tax, while un-needed desalination plants are draining the family budget.
My say: the best city to live in is where the basic human needs are easily met. It is a place where everybody is productively employed yet not overworked; where people can live in peace and know that their children and family are safe; where public servants actually serve the public and not lord over them. The list is long, but I trust you get the point.