If you're looking for a powerful tracking system, I'll recommend Tuleap[1] and esp. the "Trackers"[2]. If you need to build custom workflows or to adapt tool to an existing internal workflow, it's a tool of choice. We (I'm part of the dev team...) make a strong stand to make the tool adaptable to organizations instead of forcing orgs to adapt to the tool. Oh, and it's open source ;)

[1] https://www.tuleap.org/ [2] https://www.tuleap.org/features/issue-tracking