Each one has own features, if you want to setup a web project you should ask yourself about what kind of project is and what features you need; so in my opinion there isn’t a CMS which is better of another one.
Generally however i can list the pro and cons of each one.
WordPress is very user friendly, good for blogs and simple sites, good extendibility thanks to lots of plugins, lots of themes available.
Joomla is good for small – medium sites, communities and e-commerce, simple interface, has limit for complex jobs, some modules are commercial, good themes support.
Drupal is the max for flexibility and customization, an heaven for developers, it adapts for various tasks, many modules available, less intuitive.
Let see some statistics; according to 2010 OSCMS report we can extract:
download per week
What Are Most Sites Running On?
Any Preferred CMS and if so, which one?
You can notice the increasing popularity of WordPress also from Google Trends graph, Joomla has falling from the half 2009, whereas Drupal is stable but a more low level; another data to confirm these statistics is the graph of searched job positions, note that however here also Drupal has a big escalation.
Other interesting stats:
Joomla run from 2005 and is used by Linux.com, TNA Wrestling, Citibank, MTV Networks Quizilla.
Drupal run from 2001 and is used by Ubuntu, Le Figaro, White House, Opensource.com.
Wordpress run from 2003 and is used by TechCrunch, Playstation blog, NASA.
And you what CMS prefer and why?