That said, I've used RT (holy shit no), Pivotal Tracker (drinks too heavily from the dynamic DOM/JS koolaid, sucking up all RAM and CPU in the known universe), and JIRA. I'd take JIRA over the others.
I've used Mantis and Bugzilla and I'd also take JIRA over those.
JIRA is the best bug tracking software I've used since I left Microsoft, which was over a decade ago. Sometimes I still miss Raid, particularly it's ability to build complex queries, but also the ability to connect to any team's project. At the time, it was supposedly the only internal tool used throughout the entire company.
That said, I've used RT (holy shit no), Pivotal Tracker (drinks too heavily from the dynamic DOM/JS koolaid, sucking up all RAM and CPU in the known universe), and JIRA. I'd take JIRA over the others.