The list of countries on the link posted by pohnpei is very interesting. The only country in the top 10 that is considered a "power" country is the United States at number 9. The countries in the top 5 were a big surprise to me.
Historically, Hong Kong has been one of the most unfettered, laissez-faire countries (if you want to call it a country) in the world. It currently tops the list of most economically free countries compiled by the Heritage Foundation. The second most free country by that measure is Singapore. Both of these are said to need to be very open to business since they are tiny countries that live by their ability to attract business.
Please follow this link to see the rankings.
That's a really complicated question, and my guess is the respondents here are going to disagree, but one of my first examples would be Iraq. After the US invasion in 2003, the country's re-emerging economy became the testing ground for many modern day conservative economic ideals. The tax rate was overall quite low, a flat tax was instituted, and free trade zones carved out in many places around the country. Unlike under Saddam's rule, which practiced a complicated system of bribery and patronage, new businesses in Iraq have very few restrictions and processes to go through, and foreign investors face even less scrutiny. This is a situation that is constantly changing, however, as the new government and economic system there matures.