I personally think these ratings are useless to the general population. Choosing a city to live is like choosing a mate... you may visit several, and realize that they're "just not right". You might even try living in a few, or just take them for an extended test drive. But in the end, you go where you feel comfortable. Be it where your friends/family are, what weather and surroundings you like, what language you speak, and what sort of activities and work you do.
I tried NYC and it wasn't for me. But for a buddy of mine, he'd never live anywhere else. I have chosen Vancouver, at least for now... some things I like, some things I find frustrating. I miss Toronto's vibrant streets and interesting shops and neighbourhoods, for example. But Vancouver's weather is mild and the scenery is stunning. And then some people love Madrid... oh Madrid. So fun
Some people like architecture and well-decorated streets. Some don't care. Some people like outdoor activities, and some don't. Some like the arts, and others are into sporting events and activites. Some people like hot climates, others cooler climates. Some people like busy vibrant places, while others like the country...
So ya, take these reports with a grain of salt. Well ok maybe Bagdad's not the city for most of us. But I'm sure the top 50 or more are all excellent places to live, depending on what type of person you are. Subjectivity is HUGE when evaluating a city, in my opinion and experience.