As of the current landscape, FortiOS 7.4 is the cutting-edge branch offering the latest features. However, many organizations still standardize on the 7.2 or 7.0 branches because they are considered "Mature." Always check the "Life Cycle" page on the Fortinet website to see which branches are currently supported and which are approaching EOL.

While you can upload directly via the web GUI, for remote upgrades or unreliable connections, it is safer to host the firmware on your own TFTP server and boot from it via the console.