Concrete5 is a very simple CMS for clients to mansge. I am currently looking at ModX and use Wordpress quite a bit. I checked out Drupal, but the theming is quite difficult.
Designing ModX templates is easy compared to Joomla which I personally have never liked just because of it feels like it owns you and you design by it's strict rules not by what you actually want to design.
Modx is actually a very good CMS however I find that the Admin is not easy to use. Many of the primary functions such as adding new pages and accessed via RightClicking on elements and it does take quite a while to get used to.
For many of my clients, I can only imagine that it would cause both me and them serious headaches. So I developed my own simple cms in two versions:
1. Editing of Existing Pages.
2. Editing and Creation and Pages.
It doesn't have all the bells and whistles such as custom meta tags, the ability to add a forum none of that. But to be honest what I've found is that most clients just want to be able to edit the page content simply and perhaps add or delete a page every few months. Most of my clients though are small-medium businesses. For a larger organization that has IT people who know a thing or two, ModX would be my preference.