I don't have a favorite free site builder. When I first started out in 2006, I did use a free site builder which was Office Live. It was very limiting what could be done to the site and my SEO
was not very good at all.
I moved to WordPress after learning about its benefits to flexibility, SEO
and customization. And while the foundation for WP is free, hosting is not. However, for the inexpensive price to pay for hosting (I use Hostgator) it is all well-worth the investment. A good website is an investment in the business. From there, you can find free WP templates to use however, be wary because they can have hidden malware in the freebies.
So my suggestion is to think about hosting a WP site and buy an inexpensive template page such as Elegant Themes or Themeforest to start. Eventually you can offer WP as a service which is often in client demand.