Subdomains, subdirectories, ccTLDs or parameters? Which should you use?
While there are pros and cons to any method my personal favourite is to use the correct ccTLD. It is more work but it helps search engines identify easily what country the website is targeting.
What does seem to be quite apparent is that Google likes the parameter option the least. Bing uses this option for their country versions. Bing UK for example is http://www.bing.com/?cc=gb
Google’s guidelines for international websites can be found here: https://support.google.com/webmasters/answer/182192