What task / ticket management system do you use?

At the company I work with, we use phabricator, we used to use trac, and our CEO either wants to go back to trac or purchase licenses for Jira. We use phabricator for tickets/tasks & liquidplanner for project management. Zend Desk for technical support. Slack for minute-by-minute acknowledgement on technical support w/ devs & IT remotely.

What systems do you use, and what makes it special to you and why do you use it over the others?