WordPress is the best out-of-the-box CMS for SEO in my opinion. What I see though is a lot of WordPress sites improperly managed, for instance pertaining to the permalinks.When it comes to the best CMS for SEO, WordPress wins this title. The reasons for WordPress being best for SEO are:
Using WordPress is simple and it is because of this simplicity that the user can post content on regular basis. The search engines need updated and engaging content for the user so it gives better positioning to the website in SERPS.
WordPress gives the ability to customize the permalinks so adding valuable keywords relevant to the post helps in getting SEO benefits.
The clean code that WordPress themes offer surely gives the website an edge in SEO.
WordPress enables the user to create a website structure that is SEO friendly because of the category and tags.
The presence of built-in RSS feed in WordPress helps in submission to feed directories hence Google can find and index your post in quick time.