the answer is facebook.com
According to Alexa.com, the world's most visited website is Google.com.
In 2010, Facebook was the most visited website. SOURCE: 101 Ways To Leave A Game Show (Episode 1 on June 21, 2011)
google.com
google.co.uk
google.com
WikiPedia
www.bundesregierung.de
subway is the most visited in America from the end of 2010.
Actually Google still is the most visited website in the world. However it has been competing with Facebook lately.