With one page design we don't necessarily mean that the website consists only of one single page. It means that the website has a design that allows to display all the core contents on one page. Beside that it should be easily possible to add further contents separately. Only that way the advantages of a one page design can be balanced with necessities that might come-up in the perspective of SEO. For further details regarding „one page design“ and SEO see: SEO optimization for Parallax Scrolling Websites: Is Parallax Scroll Bad or Not?
With „best“ I mean 1) unique design/structure/approach 2) easy customization and editing 3) SEO friendly 4) fully responsive 5) offering some space for the most common contents and 6) using the newest technologies.
Alpine is a bootstrap one page parallax Drupal theme. Ultra responsive and easy to customize it is the best solutions for the Creatives, Web agency and more.
If you are looking for something different and a bit surreal, please check out this Responsive One Page Parallax Template. Surreal is a easy to use one page parallax template with lots of great features. It has been designed to make it easy to customize.