Do some research.. there are hundred threads asking the same thing here.. depends what u need it for.
there is no "best" content management system out there. we need to know your specific needs before we can give you suggestions. http://en.wikipedia.org/wiki/Comparisons_of_content_management_systems that may be a decent place for you to start your research.
bevore i the free SEO tools of this page www.webuildpages.com/tools/default.htm I am very happy with this.
i am using Joomla and its best for me! if some one objects that its non-seo then there is component available: JoomSEF to solve the problem
A lesser known one, PHP website, offers easy form to database! Adding Adsense can be a bit tricky, and the pagemaker is basic, but it gets good reviews and is called "rock solid".
I tried various CMS solutions including the ones already mentioned in this thread but I eventually settled on e107. It's not as well known but I found it to be the most user friendly of all the ones I tried. You can view my site at http://www.nemsplace.co.uk to see the kind of site it builds and if you like it, the e107 site is at http://e107.org/news.php