Lets first start with Joomla, this CMS is a very reliable I have been using it for a couple of years now. I use it mostly for building websites that have a lot of content; I also use it because it’s easier to keep track of dynamic content. Joomla can also be used for static content but I would. This has a variety of plug-ins and modules that expand everything that it does. It also has a very robust community; they are there if you run into any trouble. Some of the down side to using joomla is that it can be hard to learn in the beginning it has a bigger learning curve but once you have it there are so many powerful things that you can do with this CMS. It can also slow you website if you over use the plug-ins that it has to offer, so always keep in mind that the more plug-ins you have the slower your website is going to become.
Drupal is another great CMS, it is similar to Joomla. It loads much faster than Joomla when using the plug-ins. It's not a lot but still if people don't like to wait too long for the site to load. Drupal doesn't have as much variety when it comes to the plug-ins that they offer. It is big but not as big as that of Joomla. If you think that Joomla is hard to learn then Drupal is not for you. Talking from personal experience it is much harder to learn the way Drupal works than that of Joomla
Wordpress is mostly known for a blogging platform but it can be used as a CMS. During the past years it is moving toward the CMS realm and maybe one day it will get there. While some people don't like it because of the limitations that it has it is one of the easiest to learn. It has many plug-ins and a great community. I personally used it just for blogging but there are some people out there that used as a CMS and why not the learning curve is really small.