Recommended Firmware for F5 - i2800 (As of Feb,2018)

I think you can go with either 12.1.x or 13.1.x, since they are both Long Term Stability Releases. Which version you choose may depend on what kind of environment you are deploying this box. You can also take in to account which modules you need to provision and if your setup demands for specific functionality that is only available in 13.1.x.

The 13.1.x. version comes with new features. New feature may also come with new bugs. For most of my customers I'll recommend 12.1.x instead of 13.1.x if they don't need specific functionalty that is only present in 13.1.x.