Best at what? You really can't answer this question as there isn't a one size fit's all. I have a personal preference, but will use others depending on the task. Asking if it's SEO friendly isn't the application, it's a feature, As pointing you towards wordpress for store site or drupal for a blog doesn't really work.
You may try using Joomla? One of my friends manages all his works with Joomla. Again as Mystery man said, you may choose as per the desired task.
I totally agree with Mistery-man. How do you define the "best" CMS? If I may suggest, instead of trying to find the "best" CMS, why not try to define what do you need from a CMS and from there try to find a CMS that best suits your needs. There is no such thing as the "best" CMS. Each one of them has their own strengths and weaknesses.
For your varied "what is the best cms", it would be hard to answer. If you ask me what is my favorite cms, though, I would definitely say Wordpress. It is very easy to understand and quite easy to configure too. To fit your SEO needs, there are also various "plugins" to help you.
I would say wordpress. I have just been getting the hang of it recently and the power it has to offer is incredible! plenty of themes available if you want to design for cheap and there are plenty of designers who can integrate a complete custom design around it. It seems like there is really just no end to what it can do
maybe you should try wordpress. It's very easy to use for a beginner, and can be a powerful cms for advanced user. You can find many template/themes, and plug-in for further website development. And there is many cheap template coder out there.