As a former Technical Support Engineer, one question I was often asked was "What version of PAN-OS do you recommend?" Unsurprisingly, this question also comes up on a regular basis as a LIVEcommunity discussion.
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Recommended versions article detailing out the current recommended versions
The Support PAN-OS Software Release Guidance article is constantly updated with every new revision. It lists out all of the currently supported versions of PAN-OS, Release Dates, and what version is Support Preferred.
The article covers the following products:
PAN-OS for Firewalls
Panorama on VM / M-series
User-ID / Terminal Server Agent
That article is reviewed on a weekly basis to ensure that it remains up-to-date with the latest information on the recommended version of the latest software.