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Thought this might be a fun discussion and a way for community members to learn more about extending JIRA. Add a comment below for the single plug-in you couldn't live without and add a bit about what it does that makes it so valuable for you.
As an Adaptavist, (and a fan even if I'd not joined them), I am supposed to say Script Runner.
But, having been through hundreds of Jira systems, I can't identify one single add-on I *need* for all of them. I always install the Calendar, Jira Toolkit, and Charts as a matter of course, but some installs could live without them.
The closest I can get to "single plugin" is actually four - I haven't found a single Jira that does not gain from having the advanced workflow and automation tools provided by ScriptRunner, Automation, Suite Utilities or Misc Workflow Extensions. Generally, you only need one of them, and sometimes two. But which one depends on the Jira and the users.