Every day people ask this question, and in my opinion is not the wright question. Each content management system has his good and bad point, and when you look for a solution, you need to define the purpose of that solution. What do you expect from that cms to do for your website!
Here are the major 3, that i used in the last years.
1. Wordpress
Free, Open Source, PHP based
Wordpress is definitely the most popular and successful blogging system (which is a type of CMS), used by about 50 million blogs. Try google "powered by wordpress" and see for yourself! The CMS is also constantly being upgraded with focus on aesthetics, web standards and usability, making it the most good-looking and highly-functional blog system that even an idiot can use!
The bad part is that he can't work like a regular cms, his main purpose is for blog use, and stucks there!
So if you need a blog solution, chose Wordpress.
2. Drupal
Free, Open Source, PHP based
Drupal is a very extendable CMS, and it is able to function as any kind of website. There over 2000 modules to download and use, and a few themes. Drupal is a great all-round CMS that can be transformed into any type of website you need.
The bad part is that the templates designers are very ahrd to find, so you need to know a lot of Php to develop you're design, and also most of the changes in the website. Compared to Joomla, Drupal has very few templates to use, even the ones that requires payment, are not as decent as joomla ones. Also prepare yourself to learn a lot of php, if you start your website using Drupal.
If you are a web developer or have a good experience in php, then Drupal is a good solution for your website, and even better, for managing multiple websites, that Joomla can't do.
3. Joomla
Free, Open Source, PHP based
Joomla is popular for its large community of about 200,000 people and the availability of many extensions (there are over 4200 extensions!). Because of the number of extensions, Joomla can be used to rapidly create simple to moderately-advanced websites with absolutely no knowledge of PHP. Joomla is simply the easiest CMS to use, with coding as something optional, with all those extensions. And also the amount of decent themes for free or not, makes joomla the perfect solution for non experrience users.
The bad part is that the most used extensions, are not free, so be prepared to invest a small amount in to it.
So if you need a regular busienss website that can be easy to manage and have a lot of potential, then Joomla is for you.
In the last 2 months we start working on LifeRay. WOW
Liferay is a great Java based CMS, with huge potential, for big news portal or communities . I can't offer an opinion based by experience, because i need more time to test it.
Creative Consultant for Attitude Web Design Agency