There are several gosh. It's up to you really. I prefer wordpress or drupal. I would also use Joomla if I needed something that joomla could do compared to the others. There are Pros and Cons for any cms/software you use to power a site. Some others are CMS Made Simple or ModX or just good ol' Html. The classic debate is joomla vs wordpress vs drupal. Just find a system
you like and run with it
.
Wordpress is a great option if you have someone looking for an easy quick platform to keep information published. It's a very popular platform and there are countless tutorials. Wordpress is not that hard to customize. I might be able to help you if you headed down that route.
Joomla is based on an article system. I find that joomla is a great choice if you have a bigger site with multiple users submitting data. That's just from my experience.
Drupal is Drupal. Steep learning curve, but if you conquer it, you will be able to do anything you want. The White House's website even uses Drupal.
CMSMS is a new system that is actually pretty awesome. It's known for being simple. The theming system is the easiest I have seen. It uses something called smarty to help you theme. Where you want a page's content you would just put [content] and so on (or {content} something along those lines).