Since the very beginning (11 years ago), at Internet World Stats we have always used the ISO 3166-1 alpha-2 country codes to identify the over 233 countries we cover. The use of standards is always the best decision and the best practice. It helps you to avoid future problems.
The country codes found in ISO 3166-1 are used by many organizations, businesses and governments. For example all national postal organizations throughout the world exchange international mail in containers bearing its country code for identification. In machine readable passports, the codes from ISO 3166-1 are used to determine the nationality of the user. In addition, Internet domain name systems use the code to define top level domain names such as ‘fr’ for France, ‘au’ for Australia, ‘br’ for Brazil and so forth.