America gets the 20th century....arguably the 19th too... ;-)
...then Germany gets the four centuries from the 14th-18th...and on it goes covering a few centuries apiece for the Holy Roman Empire and the Greek City-States. China and Egypt would get a few centuries between themselves to share as well.
For such an argument to be effective, one must really nail it down to a precise definition - like so:
America is the most free nation on earth and has done more to free it's citizens and the citizens of other nations than any other nation existing on earth today.