For a basic website, Wordpress would be best in my opinion. It is by far the easiest for newbies to learn. Making templates for it is not all that difficult either. As far as basic CMSes go, Wordpress is, I believe, the most widely used.
It depends on how complex the sites are. If you have simple brochure type websites, WordPress is a great one to use but if you’re working with more complex websites, you might want to look at one of the others that have a richer interface fro customization like Drupal, Joomla, Expression Engine, Concrete5, etc…
A couple of things to keep in mind:
1) Learn how to configure the CMS to make it user friendly for your clients. This will make or break the site int he eyes of the client. I work in Drupal which has a reputation for being complex and overwhelming but I have a “recipe” for configuring it nicely for admin users. I had a client with a couple of sites come up to me the other day and say she hated her WordPress site because it was so hard to do anything compared to the Drupal site we built for her.
2) After launch support is very important. Wordpress, Joomla, Drupal, etc… sites are constantly being hit by hackers trying to get in one way or another. Make sure you offer post launch support (for a fee) and keep the site patched up and secure.
3) Get involved in the CMS community. Regardless of which type of CMS you choose, there will be a massive wealth of knowledge in the community. Get involved and soak it all in. It will make you better at what you do.
Wordpress CMS is so popular for its simplicity. Users with no coding knowledge can easily change the content. The dashboard is also user friendly.
Wordpress is best for you as your site is built on only html and css.