This is one question, like "what is the best car."
The short version is "It depends"
The slightly longer version is that the only person who can answer this is someone who understands your problem, and *all* the tools.
I have used quite a few CMS systems in my time, and they all have strengths and weaknesses. Take the big 3 (Wordpress, Joomla, Drupal). There are many cases where I recommend them. In other use cases, I might recommend using something else.
Still yet, if you have someone who has used say WP extensively, you might recommend a WP solution that is good enough, when a Joomla solution might be better. Given that they already know WP, they will get more out of it than learning Joomla.
Same thing with learning. It depends on the user. Some users are better off running WP. Others have requirements that might grow, and a plan for this, so really are better off taking the time, and making the effort to learn something like Drupal. They will start off slower, but eventually will have better skills and a better solution.
In short, asking for the best CMS without knowing the exact problems and existing skills and infrastructure of the customer, is fine as an academic exercise, but really not going to give you a useful answer.