As mentioned above, drupal and joomla have to be up there competing with it (though maybe not winning )I don't think Wordpress has a rival, to be honest. It's at the top of it's game, and by being free, nothing can compete with it.
Well, Wordpress is the best. I think Drupal and Joomla, in terms of CMS software, are also popular, but not widely used or with such great community like Wordpress
Very well said, their competitors were left behind.Rivals in what context?
Usage - Wordpress is the leader.
Ease of Use - Wordpress is much easier to upload/install, use (post content)
Add Ons - Wordpress has got millions of plugins and most of them are for free use and can be used to accomplish alost anything
Themes - there are millions of themes available - both free and premium - both normal and responsive.
This definitely leaves their nearest competitors (Joomla, Drupal and maybe many others) miles behind!