If you're looking for guidance regarding what firmware has been the most trouble-free across the breadth of documented functionality, I know 6.4.6 is solid. We've moved most of our fleet to 6.4.9, so have about 4 weeks experience on this release and thus far we have not encountered any show stoppers. I will update in 4 weeks time when we've completed the upgrade on our most complicated units.
Update: I can confirm 6.4.9 appears to work well, even on 1500D units with lots of functionality enabled (BGP, FortiWiFi, IPSec, FortiEMS, RADIUS, LDAP, SDWAN, etc.). Be sure to carefully review upgrade notes from 6.0 to 6.2 and 6.2 to 6.4. I do recall a change in SSL/TLS for LDAPS as well as removal of device groups, so if those are in use, you should prepare/adjust accordingly.
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