According to a recent article posted by Ray Clancy on the ExpatForum, "About 13% of the world’s adults, or more than 640 million people, say they would like to leave their country permanently and roughly 150 million of them say they would like to move to the United States, according to research from Gallup".
The United Kingdom and Canada are the next most popular. Australia ranks 6th, after France and Saudi Arabia.
According to the article, the Gallup poll found that the most popular reason for migration is the search for opportunity. "Opportunity could mean the chance to join family members who are already in other countries, to start a new business, to express one’s views without fear, or to live where children are treated with respect."
For more information, check out the article either from the link above or here: http://www.expatforum.com/america/the-united-states-is-the-most-popular-dream-destination-poll-reveals.html