Here's a pretty good series of pie charts that says a lot about the different cms's.
http://trends.builtwith.com/cms
Basically, wordpress is easiest to use, has lots of built in features, readily available themes and plugins and therefore has the lion's share of the whole market.
But there are clearly reasons that highly trafficked and more advanced sites use Drupal and to a lesser extent Joomla (v. v. little personal experience with these, but the trend seems pretty clear).