Do you mean:
* North America? If so, the most popular girl's name in the United States is Emma, followed by Olivia and Ava.
* South America? The most common girl's name in Brazil, the most populous country in South America, is Sophia.
* All of the Americas? It's difficult to definitively say without specific data from every country.
To give you a more precise answer, please specify which part of the Americas you're interested in.