A great Web site is both usable and aesthetically pleasing. Of course, many people would argue whether a site's usability is more, or less, important than its looks. The final answer is in your site visitors' hands, or rather their clicks. If they like your site's mix of usability, functionality and looks, your site will be successful.
But, of course, don't leave the success of your site to chance. Conduct regular tests on every aspect of your site design. Even the most successful of sites can be improved with testing.