Greg Mankiw's Blog: What nation has the most progressive tax system?

Based on these numbers, the U.S. has the most progressive tax system in the world by far.

The top 10% of earners earn about 33.5% of all income on the U.S. but pay 45.1% of taxes. They in essence pay 35% more in taxes than what they earn as a share of income.

There is also an interesting discussion on how these numbers are used and interpreted.

http://gregmankiw.blogspot.com/2011/03/what-nation-has-most-progressive-tax.html