Each Open Source CMS has it's good and bad points.  I personal use Wordpress as it can do 95% of the sites my clients need.  There's the odd site I would not be to great for.

But that's not to say it's 'the best' for yourself or clients.

It's a good CMS to start on.  Well documented and tonnes of tutorials out there.

It depends on what you want to achieve. I use wordpress for 99% of CMS needs, but recently had a large corporate client who's site would never have been able to run on WordPress, so we recommended Drupal which had alot more enterprise level features.

So, like with any range of tools, it depends on the hands holding the tool and the job at hand. Oh and maybe the budget as well.

I think Symphony is one of the best because it is literally possible to do anything with it. There are no standard fields, each section is defined by different types of custom fields that you set up. And the templating code is very easy to understand, perhaps more so than PHP.

I know a lot of people prefer Wordpress, but I don't like it for three reasons: It doesn't allow you to have multiple sections, it is more blog-oriented, and it lacks that clean, straightforward feeling in the admin panel.

Other ones that are great are Frog CMS and GetSimple. They're both very simple and revolve around pages (not posts), but Frog has blog capabilities whereas GetSimple doesn't.

In the past year I've taken away over 40 clients from other designers/developers who've used popular CMS systems such as WordPress, Joomla and Drupal because they were just too difficult for the clients to understand (and some of these client's businesses are making over $1 million in revenue a year)Believe it or not the solution was a well designed, well sturctured, tabled HTML 4.01 site that they could edit in the cPanel File Manager's HTML Editor.

Concrete5 A+

I mostly use wordpress as a cms.

By far the best CMS is concrete5. It's super easy to develop for and even easier for the end user to actually use.

I agree with Posteriouslook, but I'm quite quickly turning to Concrete5, haven't had any problems with it yet!

Since you're asking for designers and not for coders, I would strongly suggest Wordpress. You can create basic but great looking websites without too much developing skills and even if you take to time to get to know the basics of CSS, PHP and wordpress theme development, you can build about every type of site you need. Also your clients will be pleased by the easy to use and understand 'backend' of the CMS!

CMS Triangle: Wordpress-Drupal-Joomla . And there is also a powerful CMS called Chyrp.

Here you can find a roundup of the best and most used Open Source CMS
The Best Open Source CMS

Joomla is a good, solid system that has a vast community supporting it.  Can't go wrong with Joomla.

Wordpress - hands down

<full disclosure> I'm biased because I consult for these folks</full disclosure> but Webvanta appears to be more scalable than Wordpress in the long run plus I think it is pretty easy to use and built on RoR. You could try it free.

This is a tricky question, since everyone has different preferences and needs for their projects. If you're looking to design exceptional websites then I would go with WordPress, however there are a few more that are worth exploring.

Here's a quick list:

WordPress: www.wordpress.orgDrupal: www.drupal.orgFrogCMS: www.madebyfrog.comJoomla: www.joomla.orgMovableType: www.movabletype.org