What do you like best?
An easy way to create, modify and delete test cases, group them.
In general, TestRail has a very good way to structure, reuse and link the test cases, avoid redundancy and useful way to show the information.
Also, it allows control the test runs, the life cycle of the versión and the reports make this software a great tool for QA.
You can connect it with Jira and you can configure to show the test cases and bugs related between the platforms.