The best CMS
I have developed my website using Drupal. The learning curve for Drupal is a bit steep than other Content Management Systems like Joomla or word press. I initially tried developing my site using Joomla but unfortunately found most of the non-commercial modules do not work properly.
If you are planning to use PHP then I feel Drupal would a good choice. Drupal is also very SEO friendly as there are plenty of external modules to optimize your website.
You cannot call word-press as a content management system. It is just a simple blogging tool. My Drupal blog got indexed in Google very quickly and got a page rank of 3. The only problem I feel with Drupal is that it is a bit hard than other CMS. But it's worth learning it.