This would greatly depend on what you're trying to do. Both have their benefits and downfalls, however in general I would say Wordpress is more popular than Joomla.
I'd say neither is 'best', it just depends on what you want to accomplish. For a simple site or blog, i'd go with Wordpress. The documentation and user-friendliness of it is great.
For a more elaborate site, I would probably choose Joomla.
But it just depends on what you want to do