The most appropriate domain extension really depends on what your website and respectively business will be related to. ‘'.com'' is usually a great choice, especially if you want to develop your business globally. ‘'. us'' or ‘'.eu'' are great extensions if you want to focus more on the American or European market. I think it's better to pay more attention to the domain name itself, and choose a really good one, following the guidelines. As you probably know, it should be easy to remember and type. If you want it to be brand-related, you may purchase also other domain extensions, or variations of your domain name, so competitors can't take advantage of them.