Sucuri is an industry leader in website security across all platforms. Each year they create a “Hacked Website Report”. The data they share is very insightful for what CMS's are safe for creating websites.
Last year's data was just released and security issues with Joomla continue to decline going from 17% to 13% for all incidents. Wordpress infections rose from 74% to 83%. Joomla continues to be one of the most secure platforms for launching your next project.