Use the latest download available on the standard download page.
All versions Atlassian make available on the main download pages are "stable" by your definition.
We do recommend that you approach x.y.0 versions with some caution, as it is common to find they have some issues that are found when a major version goes out to a wider audience and the early adopters do a lot of testing. By all means test them, but most production deployments will be x.y.1 or above as early testing finds stuff Atlassian fix quickly.
Install Software first, and use "manage applications" to add Service Desk afterwards.
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